Step 1: Understanding the Question:

We are given a 3x3 matrix A and asked to determine its properties from the given options.

The key properties to check are whether the matrix is singular, invertible, or symmetric.


Step 2: Key Formula or Approach:

A matrix is singular if its determinant is zero (\(\det(A) = 0\)).

A matrix is invertible (or non-singular) if its determinant is non-zero (\(\det(A) \neq 0\)).

A matrix is symmetric if it is equal to its transpose (\(A = A^T\)).

We will calculate the determinant of A to check if it is singular or invertible.


Step 3: Detailed Explanation:

The given matrix is:
\[ A = \begin{bmatrix} 1 & 2 & 3
4 & 5 & 6
7 & 8 & 9 \end{bmatrix} \]

Let's calculate the determinant of A:
\[ \det(A) = 1 \begin{vmatrix} 5 & 6
8 & 9 \end{vmatrix} - 2 \begin{vmatrix} 4 & 6
7 & 9 \end{vmatrix} + 3 \begin{vmatrix} 4 & 5
7 & 8 \end{vmatrix} \]
\[ \det(A) = 1(5 \times 9 - 6 \times 8) - 2(4 \times 9 - 6 \times 7) + 3(4 \times 8 - 5 \times 7) \]
\[ \det(A) = 1(45 - 48) - 2(36 - 42) + 3(32 - 35) \]
\[ \det(A) = 1(-3) - 2(-6) + 3(-3) \]
\[ \det(A) = -3 + 12 - 9 \]
\[ \det(A) = 0 \]

Since the determinant of A is 0, the matrix is singular.

This also means the matrix is not invertible.


Now, let's check if the matrix is symmetric.

The transpose of A is:
\[ A^T = \begin{bmatrix} 1 & 4 & 7
2 & 5 & 8
3 & 6 & 9 \end{bmatrix} \]

Since \(A \neq A^T\), the matrix is not symmetric.


Step 4: Final Answer:

The determinant of the matrix A is 0. Therefore, the matrix is singular.
Quick Tip: For a 3x3 matrix with elements in an arithmetic progression like this one, the determinant is always zero. Notice that the elements in each row (1,2,3), (4,5,6), (7,8,9) and each column (1,4,7), (2,5,8), (3,6,9) are in AP. A property of determinants states that if we perform the operation \(C_2 \rightarrow C_2 - C_1\) and \(C_3 \rightarrow C_3 - C_2\), the new columns will be identical, making the determinant zero.

Step 1: Understanding the Question:

We are given a 2x2 matrix A with its determinant equal to 5.

We need to find the determinant of the matrix 2A, which is obtained by multiplying every element of A by the scalar 2.


Step 2: Key Formula or Approach:

For a square matrix A of order \(n\) and a scalar \(k\), the determinant of the matrix \(kA\) is given by the property:
\[ \det(kA) = k^n \det(A) \]


Step 3: Detailed Explanation:

We are given the following information:

The matrix A is of order \(n=2\).

The determinant of A is \(\det(A) = 5\).

We need to find the determinant of the matrix 2A.

Here, the scalar \(k\) is 2.


Using the formula from Step 2:
\[ \det(2A) = 2^n \det(A) \]

Substitute the values \(n=2\) and \(\det(A)=5\):
\[ \det(2A) = 2^2 \times 5 \]
\[ \det(2A) = 4 \times 5 \]
\[ \det(2A) = 20 \]


Step 4: Final Answer:

The determinant of the matrix 2A is 20.
Quick Tip: Remember that when a matrix is multiplied by a scalar \(k\), every element gets multiplied by \(k\). When calculating the determinant, each of the \(n\) rows (or columns) has a common factor of \(k\), which can be taken out. This results in the factor \(k\) being taken out \(n\) times, leading to the formula \(\det(kA) = k^n \det(A)\). This is a common source of error where students might mistakenly think \(\det(kA) = k \det(A)\).

Step 1: Understanding the Question:

We are dealing with a system of linear equations represented in matrix form as \(AX = B\), where A is a 3x3 coefficient matrix.

The question states that this system has a unique solution and asks about the property of the determinant of matrix A.


Step 2: Key Formula or Approach:

According to the Cramer's rule and matrix inversion method for solving systems of linear equations, a system \(AX = B\) has a unique solution if and only if the coefficient matrix A is invertible (non-singular).

A matrix is invertible if and only if its determinant is non-zero.


Step 3: Detailed Explanation:

The system of linear equations is given by \(AX = B\).

If the matrix A is invertible, we can find its inverse, \(A^{-1}\).

Multiplying the equation by \(A^{-1}\) on the left, we get:
\[ A^{-1}(AX) = A^{-1}B \]
\[ (A^{-1}A)X = A^{-1}B \]
\[ IX = A^{-1}B \]
\[ X = A^{-1}B \]

This equation gives a unique solution for the variable matrix X.


The condition for the existence of \(A^{-1}\) is that the determinant of A must be non-zero (\(\det(A) \neq 0\)).

If \(\det(A) = 0\), the matrix A is singular, and its inverse does not exist. In this case, the system of equations will have either no solution or infinitely many solutions, but not a unique solution.


Since the problem states that the system has a unique solution, it is necessary that the determinant of A is non-zero.

The value can be any non-zero real number; it is not restricted to be 1 or only positive.


Step 4: Final Answer:

For the system of equations \(AX = B\) to have a unique solution, the determinant of the coefficient matrix A must be non-zero.
Quick Tip: For a system of linear equations \(AX=B\): If \(\det(A) \neq 0\), there is a unique solution. If \(\det(A) = 0\) and \((adj A)B \neq 0\), there is no solution (inconsistent system). If \(\det(A) = 0\) and \((adj A)B = 0\), there are infinitely many solutions (consistent system). This summary is crucial for solving problems related to the nature of solutions of linear equations.

Step 1: Understanding the Question:

We are given a 2x2 matrix A containing an unknown variable \(x\), and its inverse matrix A\(^{-1}\).

We need to find the value of \(x\).


Step 2: Key Formula or Approach:

The product of a matrix and its inverse is the identity matrix, i.e., \(A A^{-1} = I\).

For a 2x2 matrix \(A = \begin{bmatrix} a & b
c & d \end{bmatrix}\), its inverse is given by \(A^{-1} = \frac{1}{\det(A)} \begin{bmatrix} d & -b
-c & a \end{bmatrix}\).

We can either use the property \(A A^{-1} = I\) or calculate the inverse of A and compare it with the given A\(^{-1}\).


Step 3: Detailed Explanation:

Method 1: Using the formula for the inverse

First, calculate the determinant of A:
\[ \det(A) = (x)(4) - (3)(2) = 4x - 6 \]

Now, find the inverse of A using the formula:
\[ A^{-1} = \frac{1}{4x-6} \begin{bmatrix} 4 & -3
-2 & x \end{bmatrix} = \begin{bmatrix} \frac{4}{4x-6} & \frac{-3}{4x-6}
\frac{-2}{4x-6} & \frac{x}{4x-6} \end{bmatrix} \]

We are given:
\[ A^{-1} = \begin{bmatrix} -2 & 1.5
1 & -0.5 \end{bmatrix} \]

By comparing the corresponding elements of the calculated inverse and the given inverse, we can set up equations. Let's compare the element in the first row, first column:
\[ \frac{4}{4x-6} = -2 \]
\[ 4 = -2(4x - 6) \]
\[ 4 = -8x + 12 \]
\[ 8x = 12 - 4 \]
\[ 8x = 8 \]
\[ x = 1 \]


Method 2: Using \(A A^{-1} = I\)
\[ A A^{-1} = \begin{bmatrix} x & 3
2 & 4 \end{bmatrix} \begin{bmatrix} -2 & 1.5
1 & -0.5 \end{bmatrix} = \begin{bmatrix} 1 & 0
0 & 1 \end{bmatrix} \]

Let's compute the element in the first row, first column of the product:
\[ (x)(-2) + (3)(1) = 1 \]
\[ -2x + 3 = 1 \]
\[ -2x = 1 - 3 \]
\[ -2x = -2 \]
\[ x = 1 \]

Both methods yield the same result.


Step 4: Final Answer:

The value of \(x\) is 1.
Quick Tip: Using the property \(A A^{-1} = I\) is often faster than calculating the inverse from scratch. You only need to calculate one or two elements of the product matrix to form an equation and solve for the unknown, saving valuable time in an exam.

Step 1: Understanding the Question:

We are given two 2x2 matrices, A and B.

We need to find the transpose of their product, \((AB)^T\).


Step 2: Key Formula or Approach:

There are two ways to solve this:

1. First, calculate the product matrix \(C = AB\). Then, find the transpose of C, which is \(C^T\).

2. Use the property of transpose: \((AB)^T = B^T A^T\). First, find the transposes of A and B, then multiply them in reverse order.

We will use the first method as it is more direct.


Step 3: Detailed Explanation:

Step 3a: Calculate the product AB
\[ A = \begin{bmatrix} 1 & 2
3 & 4 \end{bmatrix}, \quad B = \begin{bmatrix} 1 & 0
1 & 0 \end{bmatrix} \]
\[ AB = \begin{bmatrix} (1)(1) + (2)(1) & (1)(0) + (2)(0)
(3)(1) + (4)(1) & (3)(0) + (4)(0) \end{bmatrix} \]
\[ AB = \begin{bmatrix} 1 + 2 & 0 + 0
3 + 4 & 0 + 0 \end{bmatrix} \]
\[ AB = \begin{bmatrix} 3 & 0
7 & 0 \end{bmatrix} \]


Step 3b: Find the transpose of AB

The transpose of a matrix is found by interchanging its rows and columns.

Let \(C = AB = \begin{bmatrix} 3 & 0
7 & 0 \end{bmatrix}\).

Then the transpose of C is:
\[ C^T = (AB)^T = \begin{bmatrix} 3 & 7
0 & 0 \end{bmatrix} \]


Step 4: Final Answer:

The resulting matrix \((AB)^T\) is \(\begin{bmatrix} 3 & 7
0 & 0 \end{bmatrix}\).
Quick Tip: Remember the "reversal law" for the transpose of a product: \((AB)^T = B^T A^T\). This property is very useful and extends to more matrices, e.g., \((ABC)^T = C^T B^T A^T\). While direct multiplication worked well here, knowing this rule is essential for more complex problems.

Step 1: Understanding the Question:

The problem asks us to find the sum of the constants A and B in the partial fraction decomposition of the given rational expression.


Step 2: Key Formula or Approach:

To find the values of A and B, we first combine the terms on the right-hand side and then equate the numerators.
\[ \frac{A}{x-1} + \frac{B}{x+3} = \frac{A(x+3) + B(x-1)}{(x-1)(x+3)} \]

So, we have the identity:
\[ 2x+5 = A(x+3) + B(x-1) \]

We can find A and B by substituting strategic values for \(x\) (the "cover-up" method) or by comparing coefficients.


Step 3: Detailed Explanation:

We start with the identity:
\[ 2x+5 = A(x+3) + B(x-1) \]


Method 1: Cover-up Method

To find A, we substitute \(x=1\) to make the B term zero:
\[ 2(1) + 5 = A(1+3) + B(1-1) \]
\[ 7 = A(4) + B(0) \]
\[ 7 = 4A \implies A = \frac{7}{4} \]


To find B, we substitute \(x=-3\) to make the A term zero:
\[ 2(-3) + 5 = A(-3+3) + B(-3-1) \]
\[ -6 + 5 = A(0) + B(-4) \]
\[ -1 = -4B \implies B = \frac{1}{4} \]


Now, we calculate the sum A+B:
\[ A+B = \frac{7}{4} + \frac{1}{4} = \frac{8}{4} = 2 \]


Method 2: Comparing Coefficients

Expand the right side of the identity:
\[ 2x+5 = Ax + 3A + Bx - B \]
\[ 2x+5 = (A+B)x + (3A-B) \]

Now, compare the coefficients of \(x\) and the constant terms on both sides.

Comparing coefficients of \(x\):
\[ A+B = 2 \]

Comparing constant terms:
\[ 3A-B = 5 \]

From the first equation, we directly get the required value \(A+B=2\). We don't even need to solve for A and B individually.


Step 4: Final Answer:

The value of A+B is 2.
Quick Tip: When asked for a sum or combination of the constants (like A+B), always try the method of comparing coefficients first. As seen in Method 2, comparing the coefficients of the highest power of the variable (in this case, \(x\)) can directly give the answer without needing to calculate the individual values of A and B. This is a very efficient exam strategy.

Step 1: Understanding the Question:

We need to find the values of the constants A, B, and C in the partial fraction decomposition of the given rational function.


Step 2: Key Formula or Approach:

The problem is based on partial fraction decomposition. We start by setting up the identity:
\[ 3x-1 = A(x-2)(x-3) + B(x-1)(x-3) + C(x-1)(x-2) \]

The most efficient way to find A, B, and C is the "cover-up" method, where we substitute the roots of the denominator (\(x=1, x=2, x=3\)) into this identity.


Step 3: Detailed Explanation:

We use the identity: \(3x-1 = A(x-2)(x-3) + B(x-1)(x-3) + C(x-1)(x-2)\).


To find A, set \(x=1\):

Substitute \(x=1\) into the identity. The terms with B and C will become zero.
\[ 3(1) - 1 = A(1-2)(1-3) + B(0) + C(0) \]
\[ 2 = A(-1)(-2) \]
\[ 2 = 2A \]
\[ A = 1 \]


To find B, set \(x=2\):

Substitute \(x=2\) into the identity. The terms with A and C will become zero.
\[ 3(2) - 1 = A(0) + B(2-1)(2-3) + C(0) \]
\[ 6 - 1 = B(1)(-1) \]
\[ 5 = -B \]
\[ B = -5 \]


To find C, set \(x=3\):

Substitute \(x=3\) into the identity. The terms with A and B will become zero.
\[ 3(3) - 1 = A(0) + B(0) + C(3-1)(3-2) \]
\[ 9 - 1 = C(2)(1) \]
\[ 8 = 2C \]
\[ C = 4 \]


Step 4: Final Answer:

The values are \(A=1\), \(B=-5\), and \(C=4\). So, (A, B, C) = (1, -5, 4).
Quick Tip: The "cover-up" method is extremely fast for finding coefficients in partial fractions when the denominator has distinct linear factors. To find the coefficient A for a term \(\frac{A}{x-a}\), "cover-up" the \((x-a)\) factor in the original fraction's denominator and substitute \(x=a\) into the rest of the expression. For example, to find B: cover up \((x-2)\) in \(\frac{3x-1}{(x-1)(x-2)(x-3)}\) to get \(\frac{3x-1}{(x-1)(x-3)}\), and substitute \(x=2\): \(\frac{3(2)-1}{(2-1)(2-3)} = \frac{5}{(1)(-1)} = -5\).

Step 1: Understanding the Question:

We are given the value of \(\sin \theta\) and asked to find the possible values of \(\cos \theta\). The question does not specify the quadrant in which \(\theta\) lies.


Step 2: Key Formula or Approach:

The fundamental trigonometric identity relating sine and cosine is:
\[ \sin^2 \theta + \cos^2 \theta = 1 \]

We will use this identity to solve for \(\cos \theta\).


Step 3: Detailed Explanation:

We are given \(\sin \theta = \frac{3}{5}\).

Substitute this value into the Pythagorean identity:
\[ \left(\frac{3}{5}\right)^2 + \cos^2 \theta = 1 \]
\[ \frac{9}{25} + \cos^2 \theta = 1 \]

Now, solve for \(\cos^2 \theta\):
\[ \cos^2 \theta = 1 - \frac{9}{25} \]
\[ \cos^2 \theta = \frac{25 - 9}{25} \]
\[ \cos^2 \theta = \frac{16}{25} \]

Take the square root of both sides to find \(\cos \theta\):
\[ \cos \theta = \pm \sqrt{\frac{16}{25}} \]
\[ \cos \theta = \pm \frac{4}{5} \]

Since the quadrant of \(\theta\) is not specified, both positive and negative values are possible.

If \(\theta\) is in the first quadrant, \(\cos \theta = \frac{4}{5}\).

If \(\theta\) is in the second quadrant, \(\cos \theta = -\frac{4}{5}\).

Therefore, \(\cos \theta\) can be either \(\frac{4}{5}\) or \(-\frac{4}{5}\).


Step 4: Final Answer:

The possible values for \(\cos \theta\) are \(\frac{4}{5}\) or \(-\frac{4}{5}\).
Quick Tip: Always be careful when taking the square root in trigonometric problems. Unless the quadrant of the angle is specified, you must consider both the positive and negative roots. A positive \(\sin \theta\) value means the angle can be in Quadrant I (where cosine is positive) or Quadrant II (where cosine is negative).

Step 1: Understanding the Question:

We are given a trigonometric equation and the quadrant in which the angle \(\theta\) lies. We need to find the specific value of \(\cos \theta\).


Step 2: Key Formula or Approach:

First, simplify the given trigonometric equation using the reciprocal identity for cosecant: \(\csc \theta = \frac{1}{\sin \theta}\).

Then, use the simplified equation to determine the value of \(\theta\) or a trigonometric ratio related to \(\theta\).

Finally, use the information about the quadrant to find the correct value of \(\cos \theta\).


Step 3: Detailed Explanation:

Start by simplifying the given equation:
\[ \cos \theta \csc \theta = -1 \]

Substitute \(\csc \theta = \frac{1}{\sin \theta}\):
\[ \cos \theta \left(\frac{1}{\sin \theta}\right) = -1 \]
\[ \frac{\cos \theta}{\sin \theta} = -1 \]

We know that \(\frac{\cos \theta}{\sin \theta} = \cot \theta\).

So, the equation becomes:
\[ \cot \theta = -1 \]

We need to find the angle \(\theta\) in the second quadrant for which \(\cot \theta = -1\).

The reference angle for \(\cot \alpha = 1\) is \(\alpha = \frac{\pi}{4}\) or 45\(^{\circ}\).

In the second quadrant, the angle \(\theta\) is given by \(\theta = \pi - \alpha\) (in radians) or \(\theta = 180^{\circ} - \alpha\) (in degrees).
\[ \theta = \pi - \frac{\pi}{4} = \frac{3\pi}{4} \quad or \quad \theta = 180^{\circ} - 45^{\circ} = 135^{\circ} \]

Now, we need to find the value of \(\cos \theta\) for \(\theta = \frac{3\pi}{4}\).
\[ \cos\left(\frac{3\pi}{4}\right) = \cos\left(\pi - \frac{\pi}{4}\right) \]

Using the identity \(\cos(\pi - x) = -\cos(x)\):
\[ \cos\left(\frac{3\pi}{4}\right) = -\cos\left(\frac{\pi}{4}\right) = -\frac{1}{\sqrt{2}} \]

Rationalizing the denominator gives:
\[ \cos \theta = -\frac{1}{\sqrt{2}} \times \frac{\sqrt{2}}{\sqrt{2}} = -\frac{\sqrt{2}}{2} \]

This matches the condition that cosine is negative in the second quadrant.


Step 4: Final Answer:

The value of \(\cos \theta\) is \(-\frac{\sqrt{2}}{2}\).
Quick Tip: When you find a simple trigonometric ratio like \(\cot \theta = -1\), you can quickly visualize the angle. Cotangent is the ratio \(x/y\). For it to be -1, \(x = -y\). In the second quadrant, x is negative and y is positive, which fits. This corresponds to the line \(y = -x\) in the second quadrant, which makes a 45\(^{\circ}\) angle with the negative x-axis. The coordinates on the unit circle would be \((-\frac{1}{\sqrt{2}}, \frac{1}{\sqrt{2}})\), so \(\cos \theta\) is the x-coordinate, which is \(-\frac{1}{\sqrt{2}}\) or \(-\frac{\sqrt{2}}{2}\).

Step 1: Understanding the Question:

We are given the value of \(\sin \theta\) and asked to find the value of a more complex trigonometric expression.

The condition \(5 \sin \theta = 4\) implies \(\sin \theta = 4/5\), which is positive. So, \(\theta\) can be in the first or second quadrant.


Step 2: Key Formula or Approach:

First, find the values of \(\csc \theta\) and \(\cot \theta\) using \(\sin \theta = 4/5\).
\(\csc \theta = \frac{1}{\sin \theta}\).

To find \(\cot \theta\), we first need \(\cos \theta\) from the identity \(\cos^2 \theta = 1 - \sin^2 \theta\). Then \(\cot \theta = \frac{\cos \theta}{\sin \theta}\).

A simpler approach is to simplify the given expression first.


Step 3: Detailed Explanation:

Given \(\sin \theta = \frac{4}{5}\).

From this, we can immediately find \(\csc \theta\):
\[ \csc \theta = \frac{1}{\sin \theta} = \frac{1}{4/5} = \frac{5}{4} \]

Next, we find \(\cos \theta\):
\[ \cos^2 \theta = 1 - \sin^2 \theta = 1 - \left(\frac{4}{5}\right)^2 = 1 - \frac{16}{25} = \frac{9}{25} \]
\[ \cos \theta = \pm \sqrt{\frac{9}{25}} = \pm \frac{3}{5} \]

This gives two possible values for \(\cot \theta\):
\[ \cot \theta = \frac{\cos \theta}{\sin \theta} = \frac{\pm 3/5}{4/5} = \pm \frac{3}{4} \]


Let's evaluate the expression for both cases.

Case 1: \(\theta\) is in the first quadrant (\(\cot \theta = 3/4\))
\[ \frac{\csc \theta - \cot \theta}{\csc \theta + \cot \theta} = \frac{5/4 - 3/4}{5/4 + 3/4} = \frac{(5-3)/4}{(5+3)/4} = \frac{2/4}{8/4} = \frac{2}{8} = \frac{1}{4} \]

Case 2: \(\theta\) is in the second quadrant (\(\cot \theta = -3/4\))
\[ \frac{\csc \theta - \cot \theta}{\csc \theta + \cot \theta} = \frac{5/4 - (-3/4)}{5/4 + (-3/4)} = \frac{5/4 + 3/4}{5/4 - 3/4} = \frac{8/4}{2/4} = \frac{8}{2} = 4 \]

Since \(1/4\) is an option and \(4\) is not, we choose \(1/4\). Typically, in such problems without quadrant specification, the angle is assumed to be acute.


Alternative Simplification Method:

The expression can be simplified as follows:
\[ \frac{\csc \theta - \cot \theta}{\csc \theta + \cot \theta} = (\csc \theta - \cot \theta) \times \frac{1}{\csc \theta + \cot \theta} \]

Multiply numerator and denominator by \((\csc \theta - \cot \theta)\):
\[ \frac{(\csc \theta - \cot \theta)^2}{(\csc \theta + \cot \theta)(\csc \theta - \cot \theta)} = \frac{(\csc \theta - \cot \theta)^2}{\csc^2 \theta - \cot^2 \theta} \]

Using the identity \(\csc^2 \theta - \cot^2 \theta = 1\), the expression becomes:
\[ (\csc \theta - \cot \theta)^2 = \left(\frac{5}{4} - \left(\pm \frac{3}{4}\right)\right)^2 \]

If \(\cot \theta = 3/4\), we get \((5/4 - 3/4)^2 = (2/4)^2 = (1/2)^2 = 1/4\).

If \(\cot \theta = -3/4\), we get \((5/4 - (-3/4))^2 = (8/4)^2 = 2^2 = 4\).

The answer is \(1/4\).


Step 4: Final Answer:

Assuming the standard case of an acute angle, the value of the expression is \(1/4\).
Quick Tip: Simplifying the trigonometric expression before substituting values can often make the calculation easier. Here, converting the expression to \((\csc \theta - \cot \theta)^2\) simplifies the problem. Also, recognizing Pythagorean triples (like 3-4-5) can help you quickly find the values of other trig ratios. If \(\sin \theta = 4/5\) (Opp/Hyp), then Adjacent must be 3, so \(\cos \theta = \pm 3/5\) and \(\cot \theta = \pm 3/4\).

Step 1: Understanding the Question:

We are given an equation that relates a real number \(x\) to a trigonometric function \(\cos \theta\). We need to find the possible values of \(\cos \theta\).


Step 2: Key Formula or Approach:

The problem requires understanding the range of the function \(f(x) = x + \frac{1}{x}\) for real \(x\), and the range of the function \(g(\theta) = 2 \cos \theta\). The equality can only hold where their ranges overlap.


The range of \(f(x) = x + \frac{1}{x}\) can be found using the AM-GM inequality or calculus.

For \(x > 0\), by AM-GM, \(\frac{x + 1/x}{2} \ge \sqrt{x \cdot \frac{1}{x}} \implies x + \frac{1}{x} \ge 2\).

For \(x < 0\), let \(x = -y\) where \(y > 0\). Then \(x + \frac{1}{x} = -y - \frac{1}{y} = -(y + \frac{1}{y})\). Since \(y + \frac{1}{y} \ge 2\), we have \( -(y + \frac{1}{y}) \le -2\).

So, the range of \(x + \frac{1}{x}\) is \((-\infty, -2] \cup [2, \infty)\).


The range of \(\cos \theta\) is \([-1, 1]\). Therefore, the range of \(2 \cos \theta\) is \([-2, 2]\).


Step 3: Detailed Explanation:

We have the equation:
\[ x + \frac{1}{x} = 2 \cos \theta \]

Let's analyze the possible values for both sides of the equation.

The Left-Hand Side (LHS): \(y_1 = x + \frac{1}{x}\). As established, for any real \(x \neq 0\), the value of \(y_1\) must satisfy \(y_1 \ge 2\) or \(y_1 \le -2\). So, \(|y_1| \ge 2\).


The Right-Hand Side (RHS): \(y_2 = 2 \cos \theta\). Since the range of \(\cos \theta\) is \([-1, 1]\), the range of \(y_2\) is \([-2, 2]\). So, \(|y_2| \le 2\).


For the equation to hold, we must have \(y_1 = y_2\). The only values that are in both the range of \(y_1\) and the range of \(y_2\) are the boundary points where \(|y_1| \ge 2\) and \(|y_2| \le 2\) meet.

This can only happen when the value is exactly 2 or -2.

So, we must have:
\[ 2 \cos \theta = 2 \quad or \quad 2 \cos \theta = -2 \]

Solving for \(\cos \theta\):
\[ \cos \theta = 1 \quad or \quad \cos \theta = -1 \]

This can be written concisely as \(\cos \theta = \pm 1\).


Step 4: Final Answer:

The only possible values for \(\cos \theta\) are 1 and -1.
Quick Tip: This is a classic problem that tests your knowledge of the ranges of functions. Whenever you see the expression \(x + \frac{1}{x}\) for a real number \(x\), immediately recall its range: \((-\infty, -2] \cup [2, \infty)\). Comparing this with the bounded range of a trigonometric function like cosine or sine will quickly lead you to the solution at the boundary points.

Step 1: Understanding the Question:

We need to simplify the given trigonometric expression. The expression is a known identity.


Step 2: Key Formula or Approach:

We will use the algebraic identity \(a^3 + b^3 = (a+b)(a^2 - ab + b^2)\) or \(a^3 + b^3 = (a+b)^3 - 3ab(a+b)\).

Let \(a = \sin^2 \theta\) and \(b = \cos^2 \theta\).

Then the expression \(\sin^6 \theta + \cos^6 \theta\) becomes \(a^3 + b^3\).

We also know the fundamental trigonometric identity: \(\sin^2 \theta + \cos^2 \theta = 1\).


Step 3: Detailed Explanation:

Let's rewrite the expression using \(a = \sin^2 \theta\) and \(b = \cos^2 \theta\).

The expression is \(a^3 + b^3 + 3ab\). This does not immediately simplify. Let's use the second identity from Step 2.


We know that \(a+b = \sin^2 \theta + \cos^2 \theta = 1\).

Now, let's use the identity \(a^3 + b^3 = (a+b)^3 - 3ab(a+b)\).

Substitute \(a = \sin^2 \theta\) and \(b = \cos^2 \theta\):
\[ \sin^6 \theta + \cos^6 \theta = (\sin^2 \theta + \cos^2 \theta)^3 - 3(\sin^2 \theta)(\cos^2 \theta)(\sin^2 \theta + \cos^2 \theta) \]

Since \(\sin^2 \theta + \cos^2 \theta = 1\), this simplifies to:
\[ \sin^6 \theta + \cos^6 \theta = (1)^3 - 3\sin^2 \theta \cos^2 \theta (1) \]
\[ \sin^6 \theta + \cos^6 \theta = 1 - 3\sin^2 \theta \cos^2 \theta \]

Now, substitute this result back into the original expression given in the question:

Original Expression = \((\sin^6 \theta + \cos^6 \theta) + 3\sin^2 \theta \cos^2 \theta\)
\[ = (1 - 3\sin^2 \theta \cos^2 \theta) + 3\sin^2 \theta \cos^2 \theta \]
\[ = 1 \]


Step 4: Final Answer:

The value of the expression is 1.
Quick Tip: The identities for \(\sin^4 \theta + \cos^4 \theta\) and \(\sin^6 \theta + \cos^6 \theta\) are very common in competitive exams. It's useful to remember them: \(\sin^4 \theta + \cos^4 \theta = 1 - 2\sin^2 \theta \cos^2 \theta\) \(\sin^6 \theta + \cos^6 \theta = 1 - 3\sin^2 \theta \cos^2 \theta\) Memorizing these can save you derivation time during an exam.

Step 1: Understanding the Question:

We need to find the maximum possible value of the expression \(3 \cos \theta + 4 \sin \theta\).


Step 2: Key Formula or Approach:

An expression of the form \(a \cos \theta + b \sin \theta\) can be converted into the form \(R \cos(\theta - \alpha)\) or \(R \sin(\theta + \beta)\).

The range of such an expression is \([-\sqrt{a^2+b^2}, \sqrt{a^2+b^2}]\).

The maximum value is \(\sqrt{a^2+b^2}\) and the minimum value is \(-\sqrt{a^2+b^2}\).


Step 3: Detailed Explanation:

The given expression is \(3 \cos \theta + 4 \sin \theta\).

This is in the form \(a \cos \theta + b \sin \theta\) with \(a=3\) and \(b=4\).

Using the formula, the maximum value is \(\sqrt{a^2+b^2}\).

Calculate the value:
\[ \sqrt{3^2 + 4^2} = \sqrt{9 + 16} \]
\[ = \sqrt{25} \]
\[ = 5 \]

The minimum value would be -5. The entire expression ranges from -5 to 5.

Therefore, the maximum value is 5.


Derivation (for understanding):

Let \(3 = R \cos \alpha\) and \(4 = R \sin \alpha\).

Squaring and adding these gives:
\(3^2 + 4^2 = R^2 \cos^2 \alpha + R^2 \sin^2 \alpha\)
\(25 = R^2(\cos^2 \alpha + \sin^2 \alpha)\)
\(25 = R^2 \implies R = 5\) (since R is a magnitude).

The original expression becomes:
\(R \cos \alpha \cos \theta + R \sin \alpha \sin \theta = R(\cos \theta \cos \alpha + \sin \theta \sin \alpha)\)

Using the identity \(\cos(A-B) = \cos A \cos B + \sin A \sin B\), this is:
\(R \cos(\theta - \alpha) = 5 \cos(\theta - \alpha)\).

Since the maximum value of any cosine function is 1, the maximum value of \(5 \cos(\theta - \alpha)\) is \(5 \times 1 = 5\).


Step 4: Final Answer:

The maximum value of the expression \(3 \cos \theta + 4 \sin \theta\) is 5.
Quick Tip: For any expression of the form \(a \sin x + b \cos x\), immediately calculate \(\sqrt{a^2+b^2}\). This value is the maximum value (amplitude) of the resulting single trigonometric function. This is a very frequent question type, so knowing the formula for the maximum and minimum values is a must.

Step 1: Understanding the Question:

We need to solve the trigonometric equation \(\sin 5x + \sin 3x + \sin x = 0\) for a non-zero value of \(x\) in the interval \([0, \frac{\pi}{2}]\).


Step 2: Key Formula or Approach:

We will use the sum-to-product trigonometric formula:
\[ \sin A + \sin B = 2 \sin\left(\frac{A+B}{2}\right) \cos\left(\frac{A-B}{2}\right) \]

It's strategic to apply this formula to \(\sin 5x\) and \(\sin x\) to get a term involving \(3x\), which can then be factored with the middle term \(\sin 3x\).


Step 3: Detailed Explanation:

The given equation is \(\sin 5x + \sin 3x + \sin x = 0\).

Rearrange the terms: \((\sin 5x + \sin x) + \sin 3x = 0\).

Apply the sum-to-product formula to the terms in the parenthesis with \(A=5x\) and \(B=x\):
\[ \sin 5x + \sin x = 2 \sin\left(\frac{5x+x}{2}\right) \cos\left(\frac{5x-x}{2}\right) \]
\[ = 2 \sin\left(\frac{6x}{2}\right) \cos\left(\frac{4x}{2}\right) \]
\[ = 2 \sin(3x) \cos(2x) \]

Substitute this back into the equation:
\[ 2 \sin(3x) \cos(2x) + \sin(3x) = 0 \]

Factor out the common term \(\sin(3x)\):
\[ \sin(3x) (2 \cos(2x) + 1) = 0 \]

This gives two possibilities:

1) \(\sin(3x) = 0\)

2) \(2 \cos(2x) + 1 = 0 \implies \cos(2x) = -\frac{1}{2}\)


Let's solve each case for \(x\) in the interval \([0, \frac{\pi}{2}]\).

Case 1: \(\sin(3x) = 0\)

The general solution is \(3x = n\pi\), where \(n\) is an integer. So, \(x = \frac{n\pi}{3}\).

For \(n=0\), \(x = 0\). (This is a solution, but the question asks for a value other than zero).

For \(n=1\), \(x = \frac{\pi}{3}\). This value is in the interval \([0, \frac{\pi}{2}]\).

For \(n=2\), \(x = \frac{2\pi}{3}\), which is outside the interval.

So, from this case, we get the solution \(x = \frac{\pi}{3}\).


Case 2: \(\cos(2x) = -\frac{1}{2}\)

The principal value for which cosine is \(-\frac{1}{2}\) is \(\frac{2\pi}{3}\).

The general solution is \(2x = 2n\pi \pm \frac{2\pi}{3}\). So, \(x = n\pi \pm \frac{\pi}{3}\).

For \(n=0\), \(x = \pm \frac{\pi}{3}\). \(x = \frac{\pi}{3}\) is a valid solution.

For \(n=1\), \(x = \pi - \frac{\pi}{3} = \frac{2\pi}{3}\) (outside the interval) and \(x = \pi + \frac{\pi}{3} = \frac{4\pi}{3}\) (outside the interval).

This case also gives the solution \(x = \frac{\pi}{3}\).


Step 4: Final Answer:

The non-zero value of \(x\) in the given interval that satisfies the equation is \(\frac{\pi}{3}\).
Quick Tip: When solving equations with three or more sine or cosine terms, look for pairs of terms that can be combined using sum-to-product formulas to create a common factor. The choice of pairing is important. Pairing \(\sin 5x\) and \(\sin x\) was strategic because \((5x+x)/2 = 3x\), which matched the remaining term.

Step 1: Understanding the Question:

We need to find the general solution for the trigonometric equation \(\tan^2 x = 1\). This means finding all possible values of \(x\) that satisfy the equation.


Step 2: Key Formula or Approach:

The general solution for \(\tan^2 x = \tan^2 \alpha\) is given by the formula:
\[ x = n\pi \pm \alpha \]

where \(n\) is any integer.

First, we will find the principal value \(\alpha\) for which \(\tan^2 \alpha = 1\), and then apply the formula.


Step 3: Detailed Explanation:

The given equation is \(\tan^2 x = 1\).

We need to find an angle \(\alpha\) such that \(\tan^2 \alpha = 1\).

Taking the tangent of \(\frac{\pi}{4}\), we have \(\tan\left(\frac{\pi}{4}\right) = 1\).

Therefore, \(\tan^2\left(\frac{\pi}{4}\right) = (1)^2 = 1\).

So, we can choose our principal value \(\alpha = \frac{\pi}{4}\).


Now, we use the general solution formula for \(\tan^2 x = \tan^2 \alpha\):
\[ x = n\pi \pm \alpha \]

Substituting \(\alpha = \frac{\pi}{4}\), we get:
\[ x = n\pi \pm \frac{\pi}{4} \]

where \(n\) is any integer.


Alternative Method:

We can solve \(\tan^2 x = 1\) by taking the square root:
\[ \tan x = \pm \sqrt{1} \implies \tan x = 1 \quad or \quad \tan x = -1 \]

The general solution for \(\tan x = \tan \alpha\) is \(x = n\pi + \alpha\).

For \(\tan x = 1\), \(\alpha = \frac{\pi}{4}\). The solution is \(x = n\pi + \frac{\pi}{4}\).

For \(\tan x = -1\), \(\alpha = -\frac{\pi}{4}\). The solution is \(x = n\pi - \frac{\pi}{4}\).

Combining these two sets of solutions gives \(x = n\pi \pm \frac{\pi}{4}\).


Step 4: Final Answer:

The general solution of the equation is \(x = n\pi \pm \frac{\pi}{4}\), where \(n \in \mathbb{Z}\).
Quick Tip: Remember the general solution formulas for squared trigonometric functions, as they are often simpler than solving the two linear cases separately: If \(\sin^2 x = \sin^2 \alpha\), then \(x = n\pi \pm \alpha\). If \(\cos^2 x = \cos^2 \alpha\), then \(x = n\pi \pm \alpha\). If \(\tan^2 x = \tan^2 \alpha\), then \(x = n\pi \pm \alpha\). Notice that the format is the same for all three.

Step 1: Understanding the Question:

We need to evaluate a trigonometric expression involving cosine functions with arguments that are multiples of \(\frac{\pi}{17}\).


Step 2: Key Formula or Approach:

We will use the product-to-sum formula:
\[ 2 \cos A \cos B = \cos(A+B) + \cos(A-B) \]

And the property \(\cos(\pi - \theta) = -\cos(\theta)\).


Step 3: Detailed Explanation:

Let the given expression be \(E\).
\[ E = \cos\frac{5\pi}{17} + \cos\frac{7\pi}{17} + 2\cos\frac{11\pi}{17}\cos\frac{\pi}{17} \]

First, apply the product-to-sum formula to the third term:
\[ 2\cos\frac{11\pi}{17}\cos\frac{\pi}{17} = \cos\left(\frac{11\pi}{17} + \frac{\pi}{17}\right) + \cos\left(\frac{11\pi}{17} - \frac{\pi}{17}\right) \]
\[ = \cos\left(\frac{12\pi}{17}\right) + \cos\left(\frac{10\pi}{17}\right) \]

Now substitute this back into the expression for \(E\):
\[ E = \cos\frac{5\pi}{17} + \cos\frac{7\pi}{17} + \cos\frac{12\pi}{17} + \cos\frac{10\pi}{17} \]

We can rewrite the arguments of the last two terms using \(\pi\).

Notice that \(\frac{12\pi}{17} = \pi - \frac{5\pi}{17}\).

And \(\frac{10\pi}{17} = \pi - \frac{7\pi}{17}\).

Let's apply the identity \(\cos(\pi - \theta) = -\cos(\theta)\).
\[ \cos\left(\frac{12\pi}{17}\right) = \cos\left(\pi - \frac{5\pi}{17}\right) = -\cos\left(\frac{5\pi}{17}\right) \]
\[ \cos\left(\frac{10\pi}{17}\right) = \cos\left(\pi - \frac{7\pi}{17}\right) = -\cos\left(\frac{7\pi}{17}\right) \]

Now substitute these simplified terms back into the expression for \(E\):
\[ E = \cos\frac{5\pi}{17} + \cos\frac{7\pi}{17} - \cos\frac{5\pi}{17} - \cos\frac{7\pi}{17} \]

All the terms cancel each other out.
\[ E = 0 \]


Step 4: Final Answer:

The value of the expression is 0.
Quick Tip: When dealing with trigonometric sums involving fractions of \(\pi\), always look for pairs of angles that add up to \(\pi\) or \(\pi/2\). This allows you to use identities like \(\cos(\pi - \theta) = -\cos(\theta)\) or \(\cos(\pi/2 - \theta) = \sin(\theta)\) to simplify the expression. Here, recognizing \(12\pi/17\) as \(\pi - 5\pi/17\) was the key to solving the problem.

Step 1: Understanding the Question:

We are given the value of \(\sin \theta - \cos \theta\) and we need to find the value of \(\sin \theta + \cos \theta\).


Step 2: Key Formula or Approach:

This is a standard problem type that uses the relationship between \((\sin \theta - \cos \theta)^2\) and \((\sin \theta + \cos \theta)^2\).

We know that:
\((\sin \theta - \cos \theta)^2 = \sin^2 \theta - 2\sin \theta \cos \theta + \cos^2 \theta = 1 - 2\sin \theta \cos \theta\)
\((\sin \theta + \cos \theta)^2 = \sin^2 \theta + 2\sin \theta \cos \theta + \cos^2 \theta = 1 + 2\sin \theta \cos \theta\)

By adding these two equations, we get a direct relationship:
\((\sin \theta - \cos \theta)^2 + (\sin \theta + \cos \theta)^2 = 2\)


Step 3: Detailed Explanation:

Let \(x = \sin \theta + \cos \theta\). We need to find the value of \(x\).

We are given \(\sin \theta - \cos \theta = \frac{4}{5}\).

Using the identity from Step 2:
\[ (\sin \theta - \cos \theta)^2 + (\sin \theta + \cos \theta)^2 = 2 \]

Substitute the given values into this identity:
\[ \left(\frac{4}{5}\right)^2 + x^2 = 2 \]
\[ \frac{16}{25} + x^2 = 2 \]

Now, solve for \(x^2\):
\[ x^2 = 2 - \frac{16}{25} \]
\[ x^2 = \frac{50 - 16}{25} \]
\[ x^2 = \frac{34}{25} \]

Take the square root of both sides:
\[ x = \pm \sqrt{\frac{34}{25}} = \pm \frac{\sqrt{34}}{5} \]

Since one of the options is \(\frac{\sqrt{34}}{5}\), we select this value. The question doesn't provide enough information to determine the sign, but only the positive value is listed as a primary option choice.


Step 4: Final Answer:

The value of \(\sin \theta + \cos \theta\) is \(\frac{\sqrt{34}}{5}\).
Quick Tip: The identity \((\sin\theta \pm \cos\theta)^2 = 1 \pm 2\sin\theta\cos\theta\) is extremely useful. Memorizing the combined identity \((\sin\theta - \cos\theta)^2 + (\sin\theta + \cos\theta)^2 = 2\) provides a direct shortcut for problems where one expression is given and the other is asked.

Step 1: Understanding the Question:

We are asked to find the real part of a given complex number. The complex number is given as a fraction.


Step 2: Key Formula or Approach:

To find the real part, we first need to simplify the expression and write it in the standard form \(a+bi\).

First, expand the denominator \((2-i)^2\).

Then, multiply the numerator and the denominator by the conjugate of the resulting denominator to eliminate the imaginary part from the denominator.


Step 3: Detailed Explanation:

Let the complex number be \(z = \frac{1+2i}{(2-i)^2}\).

Step 3a: Simplify the denominator
\[ (2-i)^2 = 2^2 - 2(2)(i) + i^2 \]
\[ = 4 - 4i - 1 \quad (since i^2 = -1) \]
\[ = 3 - 4i \]

So, the expression becomes:
\[ z = \frac{1+2i}{3-4i} \]

Step 3b: Rationalize the denominator

To convert this into the form \(a+bi\), multiply the numerator and denominator by the conjugate of the denominator, which is \(3+4i\).
\[ z = \frac{1+2i}{3-4i} \times \frac{3+4i}{3+4i} \]
\[ z = \frac{(1+2i)(3+4i)}{(3-4i)(3+4i)} \]

Numerator:
\[ (1)(3) + (1)(4i) + (2i)(3) + (2i)(4i) = 3 + 4i + 6i + 8i^2 = 3 + 10i - 8 = -5 + 10i \]

Denominator:
\[ (3)^2 - (4i)^2 = 9 - 16i^2 = 9 - 16(-1) = 9 + 16 = 25 \]

So, the complex number is:
\[ z = \frac{-5 + 10i}{25} \]

Step 3c: Write in standard form
\[ z = \frac{-5}{25} + \frac{10i}{25} = -\frac{1}{5} + \frac{2}{5}i \]

The standard form is \(a+bi\), where \(a\) is the real part and \(b\) is the imaginary part.

Here, the real part is \(a = -\frac{1}{5}\).


Step 4: Final Answer:

The real part of the complex number is \(-\frac{1}{5}\).
Quick Tip: When simplifying complex fractions, always handle the denominator first. Expand any powers and then multiply by the conjugate to make the denominator a real number. Remember the conjugate of \(a+bi\) is \(a-bi\) and their product is always a real number: \((a+bi)(a-bi) = a^2 + b^2\).

Step 1: Understanding the Question:

We need to find the modulus of a complex number which is given in a fractional form with powers.


Step 2: Key Formula or Approach:

We will use the properties of modulus:

1. \(|z_1/z_2| = |z_1|/|z_2|\)

2. \(|z^n| = |z|^n\)

3. The modulus of a complex number \(z = a+bi\) is \(|z| = \sqrt{a^2+b^2}\).

Using these properties, we can find the modulus of the numerator and denominator separately without actually computing the complex number itself.


Step 3: Detailed Explanation:

Let \(z = \frac{(1+i)^{10}}{(2i-4)^4}\). We need to find \(|z|\).

Using the properties of modulus:
\[ |z| = \left| \frac{(1+i)^{10}}{(-4+2i)^4} \right| = \frac{|(1+i)^{10}|}{|(-4+2i)^4|} = \frac{|1+i|^{10}}{|-4+2i|^4} \]


Step 3a: Calculate the modulus of the numerator term

First, find the modulus of \(1+i\):
\[ |1+i| = \sqrt{1^2 + 1^2} = \sqrt{2} \]

Now, raise it to the power of 10:
\[ |1+i|^{10} = (\sqrt{2})^{10} = (2^{1/2})^{10} = 2^5 = 32 \]


Step 3b: Calculate the modulus of the denominator term

First, find the modulus of \(-4+2i\):
\[ |-4+2i| = \sqrt{(-4)^2 + 2^2} = \sqrt{16 + 4} = \sqrt{20} \]

We can simplify \(\sqrt{20} = \sqrt{4 \times 5} = 2\sqrt{5}\).

Now, raise it to the power of 4:
\[ |-4+2i|^4 = (\sqrt{20})^4 = (20^{1/2})^4 = 20^2 = 400 \]

Alternatively, \((2\sqrt{5})^4 = 2^4 \times (\sqrt{5})^4 = 16 \times 5^2 = 16 \times 25 = 400\).


Step 3c: Combine the results
\[ |z| = \frac{32}{400} \]

Simplify the fraction:
\[ |z| = \frac{16}{200} = \frac{8}{100} = \frac{2}{25} \]


Step 4: Final Answer:

The modulus of the complex number is \(\frac{2}{25}\). Note that modulus must be a non-negative real number, so options (B) and (D) are incorrect by definition.
Quick Tip: When asked for the modulus of a complex expression involving products, divisions, or powers, never try to simplify the complex number first. It's much faster and easier to use the properties of modulus (\(|z_1 z_2| = |z_1||z_2|\), \(|z_1/z_2| = |z_1|/|z_2|\), \(|z^n| = |z|^n\)) to calculate the modulus of each part separately and then combine them.

Step 1: Understanding the Question:

We are given the length of a chord and its perpendicular distance from the center of a circle. We need to find the diameter of the circle.


Step 2: Key Formula or Approach:

The key geometric property is that the perpendicular from the center of a circle to a chord bisects the chord. This forms a right-angled triangle with the radius of the circle as the hypotenuse, the perpendicular distance from the center as one leg, and half the length of the chord as the other leg. We can use the Pythagorean theorem: \((radius)^2 = (distance)^2 + (half of chord)^2\).


Step 3: Detailed Explanation:

Let \(r\) be the radius of the circle.

Length of the chord, \(L = 6\) cm.

Distance from the center to the chord, \(d = 4\) cm.


The perpendicular from the center bisects the chord. So, the length of half the chord is \(\frac{L}{2} = \frac{6}{2} = 3\) cm.


Now, we have a right-angled triangle with sides:

Hypotenuse = \(r\)

One leg = \(d = 4\) cm

Other leg = \(\frac{L}{2} = 3\) cm


Using the Pythagorean theorem (\(a^2 + b^2 = c^2\)):
\[ d^2 + \left(\frac{L}{2}\right)^2 = r^2 \]
\[ 4^2 + 3^2 = r^2 \]
\[ 16 + 9 = r^2 \]
\[ 25 = r^2 \]
\[ r = \sqrt{25} = 5 cm \]

The radius of the circle is 5 cm.


The question asks for the length of the diameter.

Diameter \(D = 2 \times r\).
\[ D = 2 \times 5 = 10 cm \]


Step 4: Final Answer:

The length of the diameter is 10 cm.
Quick Tip: This problem uses the very common 3-4-5 Pythagorean triple. Whenever you see a right-angled triangle with two sides as 3 and 4 in a geometry problem, the hypotenuse is almost always 5. Recognizing this can save you calculation time. Also, be careful to read the question fully; it asks for the diameter, not the radius, which is a common mistake.

Step 1: Understanding the Question:

We need to find the length of the tangent drawn from an external point \((x_1, y_1) = (5, 1)\) to a given circle.


Step 2: Key Formula or Approach:

The length of the tangent, \(L\), from an external point \((x_1, y_1)\) to a circle with the equation \(S \equiv x^2 + y^2 + 2gx + 2fy + c = 0\) is given by the formula:
\[ L = \sqrt{S_1} \]

where \(S_1\) is the value of the circle's expression when the coordinates of the point are substituted into it, i.e.,
\[ S_1 = x_1^2 + y_1^2 + 2gx_1 + 2fy_1 + c \]


Step 3: Detailed Explanation:

The given equation of the circle is \(S \equiv x^2 + y^2 + 6x - 4y - 3 = 0\).

The external point is \((x_1, y_1) = (5, 1)\).

First, we calculate \(S_1\) by substituting \(x=5\) and \(y=1\) into the equation of the circle:
\[ S_1 = (5)^2 + (1)^2 + 6(5) - 4(1) - 3 \]
\[ S_1 = 25 + 1 + 30 - 4 - 3 \]
\[ S_1 = 56 - 7 \]
\[ S_1 = 49 \]

Now, we find the length of the tangent using the formula \(L = \sqrt{S_1}\).
\[ L = \sqrt{49} \]
\[ L = 7 \]


Step 4: Final Answer:

The length of the tangent from the point (5, 1) to the circle is 7.
Quick Tip: To find the length of the tangent from a point to a circle, simply substitute the point's coordinates into the circle's equation (make sure the RHS is 0) and take the square root of the result. This is a direct and quick formula-based question common in exams.

Step 1: Understanding the Question:

We are given the length of a tangent from an external point and the distance from that point to the circle's center. We need to find the area of the circle.


Step 2: Key Formula or Approach:

The radius of a circle (\(r\)), the length of the tangent from an external point (\(L\)), and the distance from the external point to the center of the circle (\(d\)) form a right-angled triangle. The distance \(d\) is the hypotenuse.

By the Pythagorean theorem:
\[ d^2 = r^2 + L^2 \]

The area of a circle is given by the formula \(A = \pi r^2\).


Step 3: Detailed Explanation:

We are given:

Length of the tangent, \(L = 8\) cm.

Distance from the point to the center, \(d = 11\) cm.

Using the Pythagorean theorem, we can find the radius \(r\):
\[ 11^2 = r^2 + 8^2 \]
\[ 121 = r^2 + 64 \]

Solve for \(r^2\):
\[ r^2 = 121 - 64 \]
\[ r^2 = 57 \]

Now, we calculate the area of the circle, \(A = \pi r^2\).
\[ A = \pi \times 57 = 57\pi \]

To find the numerical value, we use the approximation \(\pi \approx 3.14159\).
\[ A \approx 57 \times 3.14159 \]
\[ A \approx 179.07063 cm^2 \]

This value is closest to the option 179.14 cm.


Step 4: Final Answer:

The area of the circle is \(57\pi\) cm\(^2\), which is approximately 179.14 cm\(^2\).
Quick Tip: Always visualize the geometry. The radius from the center to the point of tangency is perpendicular to the tangent line. This creates a right-angled triangle, which is key to solving many problems involving tangents to circles.

Step 1: Understanding the Question:

We are given the coordinates of the vertex and the focus of a parabola. We need to find its equation.


Step 2: Key Formula or Approach:

First, determine the orientation and the parameter 'a' of the parabola.

The vertex is \((h, k)\). The focus is \((h+a, k)\) for a parabola opening right.

The standard equation for a parabola opening horizontally with vertex at \((h, k)\) is:
\[ (y-k)^2 = 4a(x-h) \]


Step 3: Detailed Explanation:

The vertex is given as \(V(h, k) = (1, 0)\).

The focus is given as \(S = (2, 0)\).

Since the y-coordinates of the vertex and focus are the same, the axis of symmetry is horizontal (the x-axis, y=0).

The focus (x=2) is to the right of the vertex (x=1), so the parabola opens to the right.

The distance 'a' is the distance between the vertex and the focus.
\[ a = \sqrt{(2-1)^2 + (0-0)^2} = \sqrt{1^2} = 1 \]

Now we use the standard equation \((y-k)^2 = 4a(x-h)\) with \(h=1, k=0, a=1\).
\[ (y-0)^2 = 4(1)(x-1) \]
\[ y^2 = 4(x-1) \]

Expanding the equation, we get:
\[ y^2 = 4x - 4 \]


Step 4: Final Answer:

The equation of the parabola is \(y^2 = 4x - 4\).
Quick Tip: The position of the focus relative to the vertex determines the direction the parabola opens. If the focus is to the right of the vertex, it opens right. If left, it opens left. If above, it opens up. If below, it opens down. This directly tells you which standard form of the equation to use.

Step 1: Understanding the Question:

We are given the vertex and the directrix of a parabola and we need to find its focus.


Step 2: Key Formula or Approach:

The vertex of a parabola is the midpoint between its focus and its directrix.

The axis of the parabola is perpendicular to the directrix and passes through the vertex.

The distance from the vertex to the directrix is equal to the distance from the vertex to the focus. Let this distance be 'a'.


Step 3: Detailed Explanation:

The vertex is \(V(h, k) = (2, 0)\).

The directrix is the y-axis, which is the line \(x=0\).

Since the directrix is a vertical line (\(x=0\)), the axis of symmetry must be a horizontal line. Since the axis passes through the vertex (2,0), the axis is the x-axis (\(y=0\)).

The parabola opens away from the directrix. The vertex is at \(x=2\) and the directrix is at \(x=0\), so the parabola opens to the right.

The distance 'a' from the vertex to the directrix is the horizontal distance between the point (2,0) and the line \(x=0\).
\[ a = |2 - 0| = 2 \]

The focus lies on the axis of symmetry (\(y=0\)) and is at a distance 'a' from the vertex, inside the curve.

Since the parabola opens to the right, the focus will be at \((h+a, k)\).

Focus \(S = (2+2, 0) = (4, 0)\).


Step 4: Final Answer:

The focus of the parabola is at (4, 0).
Quick Tip: A quick way to find the focus is to think about the vertex as the "center". The directrix is on one side, and the focus is on the other, at an equal distance. Here, the vertex is at x=2, directrix at x=0 (2 units to the left). So, the focus must be 2 units to the right, at x=2+2=4.

Step 1: Understanding the Question:

We are given the equation of an ellipse and we need to calculate its eccentricity.


Step 2: Key Formula or Approach:

First, convert the equation of the ellipse to the standard form: \(\frac{x^2}{b^2} + \frac{y^2}{a^2} = 1\) (for a vertical ellipse) or \(\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1\) (for a horizontal ellipse), where \(a > b\).

The eccentricity \(e\) is given by the formula:
\[ e = \sqrt{1 - \frac{b^2}{a^2}} \]


Step 3: Detailed Explanation:

The given equation is \(16x^2 + 7y^2 = 112\).

To convert it to standard form, divide the entire equation by 112:
\[ \frac{16x^2}{112} + \frac{7y^2}{112} = \frac{112}{112} \]
\[ \frac{x^2}{7} + \frac{y^2}{16} = 1 \]

This is the standard form of an ellipse. We can identify \(a^2\) and \(b^2\). By convention, \(a^2\) is the larger denominator.

Here, \(a^2 = 16\) and \(b^2 = 7\).

Since \(a^2\) is under the \(y^2\) term, the major axis of the ellipse is vertical.

Now we calculate the eccentricity \(e\):
\[ e = \sqrt{1 - \frac{b^2}{a^2}} = \sqrt{1 - \frac{7}{16}} \]
\[ e = \sqrt{\frac{16-7}{16}} = \sqrt{\frac{9}{16}} \]
\[ e = \frac{3}{4} \]


Step 4: Final Answer:

The eccentricity of the ellipse is \(\frac{3}{4}\).
Quick Tip: Eccentricity of an ellipse is always between 0 and 1. If you get a value greater than or equal to 1 (like option A), you have made a calculation error, likely by swapping \(a^2\) and \(b^2\) in the formula.

Step 1: Understanding the Question:

We need to evaluate the limit of a rational function as \(x\) approaches infinity.


Step 2: Key Formula or Approach:

For a rational function \( \lim_{x\to\infty} \frac{P(x)}{Q(x)} \), where \(P(x)\) and \(Q(x)\) are polynomials, the limit is determined by the ratio of the leading terms (the terms with the highest power of x).

A more formal method is to divide both the numerator and the denominator by the highest power of \(x\) in the denominator.


Step 3: Detailed Explanation:

First, let's simplify the denominator of the expression:

Denominator = \(x^2 - 4x(1-x^2) = x^2 - 4x + 4x^3\).

So the limit is:
\[ \lim_{x\to\infty} \frac{4x^3 - x + 1}{4x^3 + x^2 - 4x} \]

The degree of the numerator polynomial is 3, and the degree of the denominator polynomial is also 3. Since the degrees are equal, the limit is the ratio of the leading coefficients.

Leading coefficient of numerator = 4.

Leading coefficient of denominator = 4.
\[ Limit = \frac{4}{4} = 1 \]

Formal Method:

The highest power of \(x\) in the expression is \(x^3\). Divide the numerator and denominator by \(x^3\):
\[ \lim_{x\to\infty} \frac{\frac{4x^3}{x^3} - \frac{x}{x^3} + \frac{1}{x^3}}{\frac{4x^3}{x^3} + \frac{x^2}{x^3} - \frac{4x}{x^3}} = \lim_{x\to\infty} \frac{4 - \frac{1}{x^2} + \frac{1}{x^3}}{4 + \frac{1}{x} - \frac{4}{x^2}} \]

As \(x \to \infty\), all terms with \(x\) in the denominator approach 0.
\[ Limit = \frac{4 - 0 + 0}{4 + 0 - 0} = \frac{4}{4} = 1 \]


Step 4: Final Answer:

The value of the limit is 1.
Quick Tip: For limits at infinity of rational functions: If degree of numerator \(<\) degree of denominator, limit is 0. If degree of numerator \(>\) degree of denominator, limit is \(\pm\infty\). If degree of numerator = degree of denominator, limit is the ratio of leading coefficients. This shortcut can save a lot of time.

Step 1: Understanding the Question:

We need to evaluate the limit of a function as \(x\) approaches 1. Direct substitution of \(x=1\) gives the indeterminate form \(\frac{0}{0}\).


Step 2: Key Formula or Approach:

We can solve this using one of three common methods for indeterminate forms:

1. Factorization: Factor the numerator and cancel common terms. The relevant identity is \(a^3-b^3 = (a-b)(a^2+ab+b^2)\).

2. L'Hôpital's Rule: Differentiate the numerator and the denominator separately and then take the limit.

3. Standard Limit Formula: Use the formula \( \lim_{x\to a} \frac{x^n - a^n}{x - a} = na^{n-1} \).


Step 3: Detailed Explanation:

Method 1: Factorization
\[ \lim_{x\to 1} \frac{x^3 - 1^3}{x - 1} = \lim_{x\to 1} \frac{(x-1)(x^2 + x \cdot 1 + 1^2)}{x - 1} \]

Cancel the \((x-1)\) term:
\[ \lim_{x\to 1} (x^2 + x + 1) \]

Now substitute \(x=1\):
\[ (1)^2 + 1 + 1 = 1 + 1 + 1 = 3 \]


Method 2: L'Hôpital's Rule

Since we have the \(\frac{0}{0}\) form, we can differentiate the numerator and denominator:
\[ \lim_{x\to 1} \frac{\frac{d}{dx}(x^3 - 1)}{\frac{d}{dx}(x - 1)} = \lim_{x\to 1} \frac{3x^2}{1} \]

Now substitute \(x=1\):
\[ \frac{3(1)^2}{1} = 3 \]


Method 3: Standard Limit Formula

The limit is in the form \( \lim_{x\to a} \frac{x^n - a^n}{x - a} \) with \(n=3\) and \(a=1\).

The result is \(na^{n-1}\):
\[ 3 \cdot (1)^{3-1} = 3 \cdot 1^2 = 3 \]


Step 4: Final Answer:

The value of the limit is 3.
Quick Tip: Recognizing the standard limit form \( \lim_{x\to a} \frac{x^n - a^n}{x - a} = na^{n-1} \) is the fastest way to solve this type of problem. It's a fundamental limit that's worth memorizing for competitive exams.

Step 1: Understanding the Question:

We need to find the derivative of the function \(y = x^x\), which is a function raised to the power of a function.


Step 2: Key Formula or Approach:

For functions of the form \(y = [f(x)]^{g(x)}\), we use logarithmic differentiation.

1. Take the natural logarithm (\(\ln\)) of both sides.

2. Use logarithm properties to simplify the expression.

3. Differentiate both sides implicitly with respect to \(x\).

4. Solve for \(\frac{dy}{dx}\).


Step 3: Detailed Explanation:

Let \(y = x^x\).

Take the natural logarithm of both sides:
\[ \ln y = \ln(x^x) \]

Using the logarithm power rule, \(\ln(a^b) = b \ln a\):
\[ \ln y = x \ln x \]

Now, differentiate both sides with respect to \(x\). We use the product rule on the right side.
\[ \frac{d}{dx}(\ln y) = \frac{d}{dx}(x \ln x) \]
\[ \frac{1}{y} \cdot \frac{dy}{dx} = \left(\frac{d}{dx}(x)\right) \cdot \ln x + x \cdot \left(\frac{d}{dx}(\ln x)\right) \]
\[ \frac{1}{y} \frac{dy}{dx} = (1) \cdot \ln x + x \cdot \left(\frac{1}{x}\right) \]
\[ \frac{1}{y} \frac{dy}{dx} = \ln x + 1 \]

To find \(\frac{dy}{dx}\), multiply both sides by \(y\):
\[ \frac{dy}{dx} = y (1 + \ln x) \]

Finally, substitute back \(y = x^x\):
\[ \frac{dy}{dx} = x^x (1 + \ln x) \]

Assuming \(\log x\) in the options represents the natural logarithm, this matches option (D).


Step 4: Final Answer:

The derivative of \(x^x\) is \(x^x(1 + \log x)\).
Quick Tip: The derivative of \(x^x\) is a standard result that's good to remember. Whenever you see a function in the form of (variable)\textsuperscript{(variable)}, logarithmic differentiation is the method to use.

Step 1: Understanding the Question:

We need to find the derivative of the inverse tangent function \(\tan^{-1}\left(\frac{x}{a}\right)\).


Step 2: Key Formula or Approach:

We will use the chain rule along with the standard derivative of the arctangent function.

Standard Derivative: \(\frac{d}{du}(\tan^{-1} u) = \frac{1}{1+u^2}\).

Chain Rule: If \(y = f(u)\) and \(u = g(x)\), then \(\frac{dy}{dx} = \frac{dy}{du} \cdot \frac{du}{dx}\).


Step 3: Detailed Explanation:

Let \(y = \tan^{-1}\left(\frac{x}{a}\right)\).

Let \(u = \frac{x}{a}\). Then \(y = \tan^{-1}(u)\).

First, find the derivative of \(y\) with respect to \(u\):
\[ \frac{dy}{du} = \frac{1}{1+u^2} \]

Next, find the derivative of \(u\) with respect to \(x\):
\[ \frac{du}{dx} = \frac{d}{dx}\left(\frac{x}{a}\right) = \frac{1}{a} \]

Now, apply the chain rule:
\[ \frac{dy}{dx} = \frac{dy}{du} \cdot \frac{du}{dx} = \left(\frac{1}{1+u^2}\right) \cdot \left(\frac{1}{a}\right) \]

Substitute \(u = \frac{x}{a}\) back into the expression:
\[ \frac{dy}{dx} = \left(\frac{1}{1 + \left(\frac{x}{a}\right)^2}\right) \cdot \left(\frac{1}{a}\right) \]

Simplify the expression:
\[ \frac{dy}{dx} = \left(\frac{1}{1 + \frac{x^2}{a^2}}\right) \cdot \left(\frac{1}{a}\right) = \left(\frac{1}{\frac{a^2+x^2}{a^2}}\right) \cdot \left(\frac{1}{a}\right) \]
\[ \frac{dy}{dx} = \left(\frac{a^2}{a^2+x^2}\right) \cdot \left(\frac{1}{a}\right) \]
\[ \frac{dy}{dx} = \frac{a}{a^2+x^2} \]


Step 4: Final Answer:

The derivative is \(\frac{a}{a^2+x^2}\).
Quick Tip: The derivative of \(\tan^{-1}\left(\frac{x}{a}\right)\) is a standard formula in calculus. It's highly recommended to memorize this result, \(\frac{a}{a^2+x^2}\), and the similar one for \(\sin^{-1}\left(\frac{x}{a}\right)\) to save time in exams.

Step 1: Understanding the Question:

We are given an infinitely nested radical function and asked to find its derivative.


Step 2: Key Formula or Approach:

The key to solving this is to recognize the repeating pattern. The entire expression under the first square root is simply the original function \(y\) itself. This allows us to write a simple implicit equation for \(y\), which can then be differentiated.


Step 3: Detailed Explanation:

The given function is:
\[ y = \sqrt{\sin x + \sqrt{\sin x + \sqrt{\sin x + \dots \infty}}} \]

We can rewrite this as:
\[ y = \sqrt{\sin x + y} \]

To remove the square root, we square both sides of the equation:
\[ y^2 = \sin x + y \]

Now, we differentiate this equation implicitly with respect to \(x\):
\[ \frac{d}{dx}(y^2) = \frac{d}{dx}(\sin x) + \frac{d}{dx}(y) \]
\[ 2y \frac{dy}{dx} = \cos x + \frac{dy}{dx} \]

Now, we need to solve for \(\frac{dy}{dx}\). Group all the \(\frac{dy}{dx}\) terms on one side:
\[ 2y \frac{dy}{dx} - \frac{dy}{dx} = \cos x \]

Factor out \(\frac{dy}{dx}\):
\[ \frac{dy}{dx} (2y - 1) = \cos x \]

Isolate \(\frac{dy}{dx}\):
\[ \frac{dy}{dx} = \frac{\cos x}{2y - 1} \]

To match the given options, we can multiply the numerator and denominator by -1:
\[ \frac{dy}{dx} = \frac{-\cos x}{-(2y - 1)} = \frac{-\cos x}{1 - 2y} \]


Step 4: Final Answer:

The derivative \(\frac{dy}{dx}\) is \(-\frac{\cos x}{1 - 2y}\).
Quick Tip: For any function of the form \(y = \sqrt{f(x) + \sqrt{f(x) + \dots}}\), the derivative follows the pattern \(\frac{dy}{dx} = \frac{f'(x)}{2y-1}\). Memorizing this general form can lead to an instant answer for such problems.

Step 1: Understanding the Question:

We need to find the slope of the normal line to the given curve (an astroid) at the specified point (1, 1).


Step 2: Key Formula or Approach:

1. Find the derivative \(\frac{dy}{dx}\) of the curve's equation using implicit differentiation. This will give the slope of the tangent (\(m_T\)).

2. Evaluate \(\frac{dy}{dx}\) at the given point (1, 1) to find the specific slope of the tangent at that point.

3. The slope of the normal (\(m_N\)) is the negative reciprocal of the slope of the tangent: \(m_N = -\frac{1}{m_T}\).


Step 3: Detailed Explanation:

The equation of the curve is \(x^{2/3} + y^{2/3} = 2\).

Differentiate both sides with respect to \(x\):
\[ \frac{d}{dx}(x^{2/3}) + \frac{d}{dx}(y^{2/3}) = \frac{d}{dx}(2) \]
\[ \frac{2}{3}x^{(2/3 - 1)} + \frac{2}{3}y^{(2/3 - 1)} \cdot \frac{dy}{dx} = 0 \]
\[ \frac{2}{3}x^{-1/3} + \frac{2}{3}y^{-1/3} \frac{dy}{dx} = 0 \]

Divide the entire equation by \(\frac{2}{3}\):
\[ x^{-1/3} + y^{-1/3} \frac{dy}{dx} = 0 \]

Solve for \(\frac{dy}{dx}\):
\[ \frac{dy}{dx} = -\frac{x^{-1/3}}{y^{-1/3}} = -\left(\frac{y}{x}\right)^{1/3} \]

This is the slope of the tangent, \(m_T\). Now, evaluate \(m_T\) at the point (1, 1):
\[ m_T = -\left(\frac{1}{1}\right)^{1/3} = -1 \]

The slope of the normal, \(m_N\), is the negative reciprocal of \(m_T\).
\[ m_N = -\frac{1}{m_T} = -\frac{1}{-1} = 1 \]


Step 4: Final Answer:

The slope of the normal to the curve at (1, 1) is 1.
Quick Tip: Be careful not to confuse the slope of the tangent with the slope of the normal. After finding \(\frac{dy}{dx}\), always remember to take the negative reciprocal if the question asks for the normal's slope. A common mistake is to stop after finding the tangent's slope.

Step 1: Understanding the Question:

We need to find the equation of the line that is tangent to the curve \(y=x^3\) at the point (1, 1).


Step 2: Key Formula or Approach:

1. Find the derivative of the function, \(\frac{dy}{dx}\), to get the formula for the slope of the tangent.

2. Evaluate the derivative at the given point to find the numerical slope, \(m\).

3. Use the point-slope form of a linear equation, \(y - y_1 = m(x - x_1)\), to find the equation of the tangent line.


Step 3: Detailed Explanation:

The curve is given by \(y = x^3\).

First, find the derivative with respect to \(x\):
\[ \frac{dy}{dx} = 3x^2 \]

This gives the slope of the tangent at any point \(x\). We need the slope at the point (1, 1). Substitute \(x=1\) into the derivative:
\[ m = 3(1)^2 = 3 \]

The slope of the tangent at (1, 1) is 3.

Now, use the point-slope form with \(m=3\) and the point \((x_1, y_1) = (1, 1)\):
\[ y - y_1 = m(x - x_1) \]
\[ y - 1 = 3(x - 1) \]

Simplify the equation:
\[ y - 1 = 3x - 3 \]

Rearrange it into the general form \(Ax+By+C=0\):
\[ 3x - y - 3 + 1 = 0 \]
\[ 3x - y - 2 = 0 \]


Step 4: Final Answer:

The equation of the tangent to the curve at (1, 1) is \(3x - y - 2 = 0\).
Quick Tip: To quickly check your answer, ensure two things: 1. The point (1,1) must satisfy the final equation. For option (C), \(3(1) - 1 - 2 = 3-3 = 0\). It works. 2. The slope of the line from the equation must match your calculated slope. For \(3x - y - 2 = 0\), the slope is \(-A/B = -3/(-1) = 3\), which matches.

Step 1: Understanding the Question:

We need to find the x-coordinate of the point where the given polynomial function has a local minimum value.


Step 2: Key Formula or Approach:

We use the second derivative test to find local minima and maxima.

1. Find the first derivative, \(f'(x)\).

2. Find the critical points by solving the equation \(f'(x) = 0\).

3. Find the second derivative, \(f''(x)\).

4. For each critical point \(c\), evaluate \(f''(c)\).

- If \(f''(c) > 0\), the function has a local minimum at \(x=c\).

- If \(f''(c) < 0\), the function has a local maximum at \(x=c\).


Step 3: Detailed Explanation:

The function is \(f(x) = 2x^3 + 3x^2 - 36x + 10\).

1. Find the first derivative:
\[ f'(x) = \frac{d}{dx}(2x^3 + 3x^2 - 36x + 10) = 6x^2 + 6x - 36 \]

2. Find the critical points by setting \(f'(x) = 0\):
\[ 6x^2 + 6x - 36 = 0 \]

Divide by 6 to simplify:
\[ x^2 + x - 6 = 0 \]

Factor the quadratic equation:
\[ (x+3)(x-2) = 0 \]

The critical points are \(x = -3\) and \(x = 2\).

3. Find the second derivative:
\[ f''(x) = \frac{d}{dx}(6x^2 + 6x - 36) = 12x + 6 \]

4. Apply the second derivative test for each critical point:

For \(x = -3\):
\[ f''(-3) = 12(-3) + 6 = -36 + 6 = -30 \]

Since \(f''(-3) < 0\), there is a local maximum at \(x = -3\).

For \(x = 2\):
\[ f''(2) = 12(2) + 6 = 24 + 6 = 30 \]

Since \(f''(2) > 0\), there is a local minimum at \(x = 2\).


Step 4: Final Answer:

The function has a minimum at the value \(x = 2\).
Quick Tip: For a cubic polynomial with two critical points, the smaller x-value (more negative) typically corresponds to the local maximum, and the larger x-value corresponds to the local minimum, assuming a positive leading coefficient. This can be a quick check.

Step 1: Understanding the Question:

We are given a function \(z\) of two variables, \(x\) and \(y\), and we need to evaluate an expression involving its partial derivatives.


Step 2: Key Formula or Approach:

1. Find the partial derivative of \(z\) with respect to \(x\), \(\frac{\partial z}{\partial x}\), by treating \(y\) as a constant.

2. Find the partial derivative of \(z\) with respect to \(y\), \(\frac{\partial z}{\partial y}\), by treating \(x\) as a constant.

3. Substitute these derivatives into the given expression and simplify.


Step 3: Detailed Explanation:

The given function is \(z = x^2 - y^2\).

1. Calculate the partial derivative with respect to \(x\):
\[ \frac{\partial z}{\partial x} = \frac{\partial}{\partial x}(x^2 - y^2) = 2x - 0 = 2x \]

2. Calculate the partial derivative with respect to \(y\):
\[ \frac{\partial z}{\partial y} = \frac{\partial}{\partial y}(x^2 - y^2) = 0 - 2y = -2y \]

3. Now, substitute these results into the given expression:
\[ \frac{1}{x} \frac{\partial z}{\partial x} + \frac{1}{y} \frac{\partial z}{\partial y} = \frac{1}{x} (2x) + \frac{1}{y} (-2y) \]

Simplify the expression:
\[ = 2 - 2 = 0 \]


Step 4: Final Answer:

The value of the expression is 0.
Quick Tip: This problem type is a straightforward application of partial differentiation rules. Remember that when differentiating with respect to one variable, all other variables are treated as constants.

Step 1: Understanding the Question:

We need to find the sum of the second partial derivatives of the function \(u=e^{xy}\) with respect to \(x\) and \(y\), and then evaluate this sum at the point (1, 1).


Step 2: Key Formula or Approach:

1. Find the first partial derivative \(\frac{\partial u}{\partial x}\).

2. Find the second partial derivative \(\frac{\partial^2 u}{\partial x^2}\) by differentiating \(\frac{\partial u}{\partial x}\) with respect to \(x\).

3. Find the first partial derivative \(\frac{\partial u}{\partial y}\).

4. Find the second partial derivative \(\frac{\partial^2 u}{\partial y^2}\) by differentiating \(\frac{\partial u}{\partial y}\) with respect to \(y\).

5. Add the two second derivatives and substitute \(x=1, y=1\).


Step 3: Detailed Explanation:

The function is \(u = e^{xy}\).

Derivatives with respect to x:

First partial derivative w.r.t. x (using chain rule, treat y as constant):
\[ \frac{\partial u}{\partial x} = e^{xy} \cdot \frac{\partial}{\partial x}(xy) = y e^{xy} \]

Second partial derivative w.r.t. x (treat y as constant):
\[ \frac{\partial^2 u}{\partial x^2} = \frac{\partial}{\partial x}(y e^{xy}) = y \cdot \frac{\partial}{\partial x}(e^{xy}) = y \cdot (y e^{xy}) = y^2 e^{xy} \]


Derivatives with respect to y:

First partial derivative w.r.t. y (using chain rule, treat x as constant):
\[ \frac{\partial u}{\partial y} = e^{xy} \cdot \frac{\partial}{\partial y}(xy) = x e^{xy} \]

Second partial derivative w.r.t. y (treat x as constant):
\[ \frac{\partial^2 u}{\partial y^2} = \frac{\partial}{\partial y}(x e^{xy}) = x \cdot \frac{\partial}{\partial y}(e^{xy}) = x \cdot (x e^{xy}) = x^2 e^{xy} \]


Sum of second derivatives:
\[ \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} = y^2 e^{xy} + x^2 e^{xy} = (x^2 + y^2)e^{xy} \]

Now, evaluate this expression at the point (1, 1):
\[ \left. \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} \right|_{(1,1)} = (1^2 + 1^2)e^{(1)(1)} = (1+1)e^1 = 2e \]


Step 4: Final Answer:

The value of the expression at (1, 1) is 2e.
Quick Tip: When performing partial differentiation, be methodical. Write down each step clearly to avoid confusion, especially when calculating second-order derivatives. Pay close attention to which variable you are treating as a constant in each step.

Step 1: Understanding the Question:

We need to find the indefinite integral of the function \((\log \sec x) \tan x\).


Step 2: Key Formula or Approach:

This integral can be solved using the method of substitution. We look for a function and its derivative within the integrand.

Let's test the substitution \(t = \log \sec x\). We need to find its derivative, \(\frac{dt}{dx}\).


Step 3: Detailed Explanation:

Let \(I = \int (\log \sec x) \tan x \,dx\).

Let's use the substitution \(t = \log \sec x\).

Now, we differentiate \(t\) with respect to \(x\) to find \(dt\):
\[ \frac{dt}{dx} = \frac{d}{dx}(\log \sec x) \]

Using the chain rule, let \(u = \sec x\), so \(t = \log u\).
\[ \frac{dt}{dx} = \frac{dt}{du} \cdot \frac{du}{dx} = \frac{1}{u} \cdot (\sec x \tan x) = \frac{1}{\sec x} \cdot (\sec x \tan x) = \tan x \]

So, we have \(\frac{dt}{dx} = \tan x\), which means \(dt = \tan x \,dx\).

Now we can substitute \(t\) and \(dt\) back into the integral:
\[ I = \int (\log \sec x) (\tan x \,dx) = \int t \,dt \]

This is a standard integral:
\[ \int t \,dt = \frac{t^2}{2} + c \]

Finally, substitute back \(t = \log \sec x\):
\[ I = \frac{(\log \sec x)^2}{2} + c = \frac{1}{2}(\log \sec x)^2 + c \]


Step 4: Final Answer:

The value of the integral is \(\frac{1}{2}(\log \sec x)^2 + c\).
Quick Tip: In substitution integrals, look for a composite function. The derivative of the "inner" function is often present as a factor in the integrand. Here, the inner function is \(\log(\sec x)\), and its derivative, \(\tan x\), is conveniently the other factor. Recognizing this pattern is key.

Step 1: Understanding the Question:

We need to find the indefinite integral of \(\sin^2 x\).


Step 2: Key Formula or Approach:

We cannot integrate \(\sin^2 x\) directly. We must first reduce the power using a trigonometric identity. The relevant power-reduction formula comes from the double-angle identity for cosine:
\[ \cos(2x) = 1 - 2\sin^2 x \]

Rearranging this formula to solve for \(\sin^2 x\), we get:
\[ \sin^2 x = \frac{1 - \cos(2x)}{2} \]


Step 3: Detailed Explanation:

Let \(I = \int \sin^2 x \,dx\).

Substitute the power-reduction formula into the integral:
\[ I = \int \frac{1 - \cos(2x)}{2} \,dx \]

We can split this into two separate integrals:
\[ I = \frac{1}{2} \int (1 - \cos(2x)) \,dx = \frac{1}{2} \left( \int 1 \,dx - \int \cos(2x) \,dx \right) \]

Integrate each term:
\[ \int 1 \,dx = x \]
\[ \int \cos(2x) \,dx = \frac{\sin(2x)}{2} \]

Now, substitute these back into the expression for \(I\):
\[ I = \frac{1}{2} \left( x - \frac{\sin(2x)}{2} \right) + c \]

Distribute the \(\frac{1}{2}\):
\[ I = \frac{x}{2} - \frac{\sin(2x)}{4} + c \]


Step 4: Final Answer:

The integral of \(\sin^2 x\) is \(\frac{x}{2} - \frac{\sin 2x}{4} + c\).
Quick Tip: Memorize the power-reduction formulas for \(\sin^2 x\) and \(\cos^2 x\), as they are fundamental for integrating even powers of sine and cosine. \(\sin^2 x = \frac{1 - \cos(2x)}{2}\) \(\cos^2 x = \frac{1 + \cos(2x)}{2}\) These are direct applications of the \(\cos(2x)\) double-angle identities.

Step 1: Understanding the Question:

We need to evaluate the indefinite integral of \(\frac{1}{25 - x^2}\).


Step 2: Key Formula or Approach:

This integral matches the standard integration formula:
\[ \int \frac{dx}{a^2 - x^2} = \frac{1}{2a} \ln \left| \frac{a+x}{a-x} \right| + C \]

Alternatively, we can use partial fraction decomposition.


Step 3: Detailed Explanation:

Method 1: Using the Standard Formula

The integral is \(\int \frac{dx}{25 - x^2}\).

We can write this as \(\int \frac{dx}{5^2 - x^2}\).

This matches the standard form with \(a=5\).

Applying the formula:
\[ \int \frac{dx}{5^2 - x^2} = \frac{1}{2(5)} \ln \left| \frac{5+x}{5-x} \right| + c \]
\[ = \frac{1}{10} \ln \left| \frac{5+x}{5-x} \right| + c \]

Assuming \(\log\) in the options means natural logarithm (\(\ln\)), this matches option (C).


Method 2: Partial Fraction Decomposition

Factor the denominator: \(25 - x^2 = (5-x)(5+x)\).

Decompose the fraction:
\[ \frac{1}{(5-x)(5+x)} = \frac{A}{5-x} + \frac{B}{5+x} \]

Using the cover-up method:

To find A, cover \((5-x)\) and set \(x=5\): \(A = \frac{1}{5+5} = \frac{1}{10}\).

To find B, cover \((5+x)\) and set \(x=-5\): \(B = \frac{1}{5-(-5)} = \frac{1}{10}\).

So the integral becomes:
\[ \int \left( \frac{1/10}{5-x} + \frac{1/10}{5+x} \right) dx = \frac{1}{10} \int \frac{dx}{5-x} + \frac{1}{10} \int \frac{dx}{5+x} \]
\[ = \frac{1}{10} (-\ln|5-x|) + \frac{1}{10} (\ln|5+x|) + c \]
\[ = \frac{1}{10} (\ln|5+x| - \ln|5-x|) + c \]

Using the logarithm property \(\ln a - \ln b = \ln(a/b)\):
\[ = \frac{1}{10} \ln \left| \frac{5+x}{5-x} \right| + c \]


Step 4: Final Answer:

The result of the integration is \(\frac{1}{10} \log \left| \frac{5+x}{5-x} \right| + c\).
Quick Tip: Memorizing the standard integral forms for \(\frac{1}{a^2-x^2}\), \(\frac{1}{x^2-a^2}\), and \(\frac{1}{a^2+x^2}\) is essential for speed and accuracy in competitive exams. They appear very frequently.

Step 1: Understanding the Question:

We need to evaluate a definite integral. The integrand has a term \((1-x)^9\) which is difficult to expand.


Step 2: Key Formula or Approach:

We can use a property of definite integrals or substitution.

Property: \(\int_0^a f(x) \,dx = \int_0^a f(a-x) \,dx\).

Substitution: Let \(u = 1-x\).

We will use the property as it is often faster.


Step 3: Detailed Explanation:

Let \(I = \int_0^1 x(1-x)^9 \,dx\).

Using the property \(\int_0^a f(x) \,dx = \int_0^a f(a-x) \,dx\) with \(a=1\):

Here, \(f(x) = x(1-x)^9\).

So, \(f(a-x) = f(1-x) = (1-x)(1-(1-x))^9 = (1-x)(x)^9 = x^9(1-x)\).

Therefore, the integral becomes:
\[ I = \int_0^1 x^9(1-x) \,dx \]

This is much easier to integrate as we just need to expand the integrand:
\[ I = \int_0^1 (x^9 - x^{10}) \,dx \]

Now, perform the integration:
\[ I = \left[ \frac{x^{10}}{10} - \frac{x^{11}}{11} \right]_0^1 \]

Evaluate at the limits:
\[ I = \left( \frac{1^{10}}{10} - \frac{1^{11}}{11} \right) - \left( \frac{0^{10}}{10} - \frac{0^{11}}{11} \right) \]
\[ I = \left( \frac{1}{10} - \frac{1}{11} \right) - (0) \]

Find a common denominator:
\[ I = \frac{11 - 10}{110} = \frac{1}{110} \]


Step 4: Final Answer:

The value of the definite integral is \(\frac{1}{110}\).
Quick Tip: The property \(\int_0^a f(x) \,dx = \int_0^a f(a-x) \,dx\) is extremely useful for definite integrals where the integrand involves a term like \((a-x)^n\). It often simplifies the integrand significantly, making integration straightforward.

Step 1: Understanding the Question:

We need to evaluate the definite integral of the absolute value function, \(|x|\), over a symmetric interval \([-a, a]\).


Step 2: Key Formula or Approach:

The function \(f(x) = |x|\) is an even function, because \(f(-x) = |-x| = |x| = f(x)\).

For any even function, we have the property:
\[ \int_{-a}^{a} f(x) \,dx = 2 \int_{0}^{a} f(x) \,dx \]

We also need the definition of \(|x|\): \(|x| = x\) for \(x \ge 0\).


Step 3: Detailed Explanation:

Let \(I = \int_{-a}^{a} |x| \,dx\).

Since \(|x|\) is an even function, we can simplify the integral:
\[ I = 2 \int_{0}^{a} |x| \,dx \]

In the interval \([0, a]\), \(x\) is non-negative, so \(|x| = x\).

The integral becomes:
\[ I = 2 \int_{0}^{a} x \,dx \]

Now, we evaluate this simple integral:
\[ I = 2 \left[ \frac{x^2}{2} \right]_0^a \]
\[ I = 2 \left( \frac{a^2}{2} - \frac{0^2}{2} \right) \]
\[ I = 2 \left( \frac{a^2}{2} \right) = a^2 \]


Alternative Method (Splitting the integral):

We can split the integral based on the definition of \(|x|\):
\(|x| = -x\) for \(x < 0\) and \(|x| = x\) for \(x \ge 0\).
\[ I = \int_{-a}^{0} |x| \,dx + \int_{0}^{a} |x| \,dx \]
\[ I = \int_{-a}^{0} (-x) \,dx + \int_{0}^{a} x \,dx \]
\[ I = \left[ -\frac{x^2}{2} \right]_{-a}^{0} + \left[ \frac{x^2}{2} \right]_{0}^{a} \]
\[ I = \left( -\frac{0^2}{2} - \left(-\frac{(-a)^2}{2}\right) \right) + \left( \frac{a^2}{2} - \frac{0^2}{2} \right) \]
\[ I = \left( 0 + \frac{a^2}{2} \right) + \left( \frac{a^2}{2} - 0 \right) = \frac{a^2}{2} + \frac{a^2}{2} = a^2 \]


Step 4: Final Answer:

The value of the integral is \(a^2\).
Quick Tip: Recognizing whether a function is even or odd is a powerful shortcut for definite integrals over symmetric intervals like \([-a, a]\). If \(f(x)\) is even, \(\int_{-a}^{a} f(x) \,dx = 2 \int_{0}^{a} f(x) \,dx\). If \(f(x)\) is odd, \(\int_{-a}^{a} f(x) \,dx = 0\). This can simplify the calculation significantly.

Step 1: Understanding the Question:

We need to evaluate a definite integral involving trigonometric functions.


Step 2: Key Formula or Approach:

The integrand looks complicated. We should try to simplify it using trigonometric identities. The key is to relate the numerator \(\cos 2x\) to the denominator \(\sin x + \cos x\).

The relevant identity for the numerator is the difference of squares form:
\[ \cos 2x = \cos^2 x - \sin^2 x \]

This can be factored as \((\cos x - \sin x)(\cos x + \sin x)\).


Step 3: Detailed Explanation:

Let \(I = \int_{0}^{\pi/2} \frac{\cos 2x}{\sin x + \cos x} \,dx\).

Substitute the identity for \(\cos 2x\) into the integral:
\[ I = \int_{0}^{\pi/2} \frac{\cos^2 x - \sin^2 x}{\sin x + \cos x} \,dx \]

Factor the numerator as a difference of squares:
\[ I = \int_{0}^{\pi/2} \frac{(\cos x - \sin x)(\cos x + \sin x)}{\sin x + \cos x} \,dx \]

Assuming \(\sin x + \cos x \neq 0\) in the interval \((0, \pi/2)\), we can cancel the common term:
\[ I = \int_{0}^{\pi/2} (\cos x - \sin x) \,dx \]

Now, we can integrate this simplified expression:
\[ I = \left[ \sin x - (-\cos x) \right]_{0}^{\pi/2} \]
\[ I = \left[ \sin x + \cos x \right]_{0}^{\pi/2} \]

Evaluate at the upper and lower limits:
\[ I = \left( \sin\left(\frac{\pi}{2}\right) + \cos\left(\frac{\pi}{2}\right) \right) - (\sin(0) + \cos(0)) \]
\[ I = (1 + 0) - (0 + 1) \]
\[ I = 1 - 1 = 0 \]


Step 4: Final Answer:

The value of the definite integral is 0.
Quick Tip: When faced with a trigonometric fraction in an integral, always look for identities that can simplify the expression. The double angle formulas, especially \(\cos 2x\), are very versatile and have multiple forms (\(\cos^2x - \sin^2x\), \(2\cos^2x - 1\), \(1 - 2\sin^2x\)). Choosing the right form is key. Here, the difference of squares form was perfect for cancellation.

Step 1: Understanding the Question:

We need to find the area of the region enclosed by the parabola \(y=4x^2\), the x-axis (\(y=0\)), and the vertical lines \(x=0\) and \(x=1\).


Step 2: Key Formula or Approach:

The area under a curve \(y=f(x)\) from \(x=a\) to \(x=b\) is given by the definite integral:
\[ A = \int_{a}^{b} f(x) \,dx \]

We must ensure that \(f(x) \ge 0\) in the interval \([a,b]\).


Step 3: Detailed Explanation:

The function is \(f(x) = 4x^2\).

The boundaries are \(a=0\) and \(b=1\).

In the interval \([0, 1]\), \(x^2\) is always non-negative, so the curve \(y=4x^2\) is above the x-axis.

We set up the definite integral for the area:
\[ A = \int_{0}^{1} 4x^2 \,dx \]

Now, we evaluate the integral:
\[ A = 4 \int_{0}^{1} x^2 \,dx \]
\[ A = 4 \left[ \frac{x^3}{3} \right]_0^1 \]

Evaluate at the limits:
\[ A = 4 \left( \frac{1^3}{3} - \frac{0^3}{3} \right) \]
\[ A = 4 \left( \frac{1}{3} - 0 \right) = \frac{4}{3} \]

The area is \(\frac{4}{3}\) square units.


Step 4: Final Answer:

The area of the bounded region is \(\frac{4}{3}\).
Quick Tip: Finding the area under a curve is a direct application of definite integration. Always check if the function is above or below the x-axis in the given interval. If the function dips below the x-axis, you'll need to split the integral and take the absolute value of the negative parts to get the total area.

Step 1: Understanding the Question:

We need to calculate the Root Mean Square (RMS) value of the function \(f(x) = x^2\) over the interval \([0, 1]\).


Step 2: Key Formula or Approach:

The RMS value of a function \(f(x)\) over the interval \([a, b]\) is given by the formula:
\[ RMS = \sqrt{\frac{1}{b-a} \int_a^b [f(x)]^2 \,dx} \]


Step 3: Detailed Explanation:

Here, the function is \(f(x) = x^2\) and the interval is \([a, b] = [0, 1]\).

First, we find the square of the function:
\[ [f(x)]^2 = (x^2)^2 = x^4 \]

Next, we calculate the mean square value, which is the average of the squared function over the interval.
\[ Mean Square = \frac{1}{1-0} \int_0^1 x^4 \,dx = \int_0^1 x^4 \,dx \]

Evaluate the integral:
\[ \int_0^1 x^4 \,dx = \left[ \frac{x^5}{5} \right]_0^1 = \frac{1^5}{5} - \frac{0^5}{5} = \frac{1}{5} \]

Finally, the RMS value is the square root of the mean square value:
\[ RMS = \sqrt{\frac{1}{5}} = \frac{1}{\sqrt{5}} \]


Step 4: Final Answer:

The RMS value of \(x^2\) in the interval [0, 1] is \(\frac{1}{\sqrt{5}}\).
Quick Tip: Remember the three steps for RMS: \textbf{S}quare the function, find the \textbf{M}ean (average) of the result over the interval, and then take the square \textbf{R}oot. Following the steps in reverse order (S-M-R) helps in remembering the process.

Step 1: Understanding the Question:

We need to find the degree of the given differential equation. The degree is the highest power of the highest order derivative after the equation has been made free of radicals and fractions with respect to its derivatives.


Step 2: Key Formula or Approach:

To find the degree, we must first clear any fractions or radicals involving the derivatives. The given equation has a derivative \(y'\) in the denominator.


Step 3: Detailed Explanation:

The given differential equation is:
\[ y' + y = \frac{5}{y'} \]

To eliminate the fraction, we multiply the entire equation by \(y'\):
\[ y'(y' + y) = y'\left(\frac{5}{y'}\right) \]
\[ (y')^2 + y \cdot y' = 5 \]

The equation is now a polynomial in terms of its derivatives.

First, identify the order of the equation. The highest order derivative present is \(y'\) (or \(\frac{dy}{dx}\)), so the order is 1.

Next, identify the degree. The degree is the highest power of the highest order derivative. In this equation, the highest power of \(y'\) is 2.

Therefore, the degree of the differential equation is 2.


Step 4: Final Answer:

The degree of the given differential equation is 2.
Quick Tip: Don't be tempted to state the degree by just looking at the initial form of the equation. Always clear denominators and radicals involving any derivative terms before determining the degree. The order can be found from the original equation, but the degree requires this simplification step.

Step 1: Understanding the Question:

We need to find the order of the differential equation that corresponds to the given general solution.


Step 2: Key Formula or Approach:

The order of a differential equation is equal to the number of independent arbitrary constants in its general solution. To form the differential equation, we need to differentiate the solution as many times as there are constants and then eliminate the constants.


Step 3: Detailed Explanation:

The given general solution is:
\[ y = a \sin x + b \cos x \]

This solution contains two independent arbitrary constants, \(a\) and \(b\).

According to the rule, the order of the differential equation must be equal to the number of these constants.

Therefore, the order of the differential equation is 2.


Derivation (for verification):

1. Differentiate the solution with respect to \(x\):
\[ \frac{dy}{dx} = a \cos x - b \sin x \]

2. Differentiate a second time to eliminate the constants:
\[ \frac{d^2y}{dx^2} = -a \sin x - b \cos x \]

Notice that the right side is the negative of the original expression for \(y\).
\[ \frac{d^2y}{dx^2} = -(a \sin x + b \cos x) = -y \]

So, the differential equation is:
\[ \frac{d^2y}{dx^2} + y = 0 \]

The highest order derivative in this equation is the second derivative, so the order is 2. This confirms our initial conclusion.


Step 4: Final Answer:

The order of the differential equation is 2.
Quick Tip: A very quick way to solve this type of problem is to simply count the number of independent arbitrary constants in the general solution. This count directly gives you the order of the differential equation.

Step 1: Understanding the Question:

We need to classify the given first-order differential equation into one of the standard types.


Step 2: Key Formula or Approach:

We need to check if the equation can be rearranged into the standard forms for each type:

- Variable Separable: Can it be written as \(f(y)dy = g(x)dx\)?

- Homogeneous: Can \(\frac{dy}{dx}\) be expressed as a function of \(\frac{y}{x}\)?

- Linear: Can it be written as \(\frac{dy}{dx} + P(x)y = Q(x)\)?

- Exact: Can it be written as \(M(x,y)dx + N(x,y)dy = 0\) where \(\frac{\partial M}{\partial y} = \frac{\partial N}{\partial x}\)?


Step 3: Detailed Explanation:

The given equation is \(\frac{dy}{dx} = -\frac{x+y}{1+x^2}\).

Let's split the fraction on the right-hand side:
\[ \frac{dy}{dx} = -\frac{x}{1+x^2} - \frac{y}{1+x^2} \]

Now, let's rearrange the terms to see if it matches the linear form by moving the term with y to the left side:
\[ \frac{dy}{dx} + \frac{1}{1+x^2}y = -\frac{x}{1+x^2} \]

This equation is exactly in the standard form of a first-order linear differential equation, \(\frac{dy}{dx} + P(x)y = Q(x)\), where:

- \(P(x) = \frac{1}{1+x^2}\)

- \(Q(x) = -\frac{x}{1+x^2}\)

It is not variable separable as we cannot group all \(x\) terms with \(dx\) and all \(y\) terms with \(dy\). It is also not homogeneous. Therefore, the correct classification is a first-order linear equation.


Step 4: Final Answer:

The differential equation is a First order Linear equation.
Quick Tip: When classifying a differential equation, always try to rearrange it into the standard linear form \(\frac{dy}{dx} + P(x)y = Q(x)\) first, as it's a very common type. If terms can be separated into functions of only x and only y, it's linear.

Step 1: Understanding the Question:

We need to find the general solution of the given first-order differential equation.


Step 2: Key Formula or Approach:

The equation is of the variable separable type. We will separate the terms involving \(y\) and \(dy\) from the terms involving \(x\) and \(dx\), and then integrate both sides. The key integral required is \(\int \frac{1}{1+y^2} dy = \tan^{-1}(y)\).


Step 3: Detailed Explanation:

The differential equation is:
\[ \frac{dy}{dx} = 1 + y^2 \]

Separate the variables by multiplying by \(dx\) and dividing by \((1+y^2)\):
\[ \frac{dy}{1+y^2} = dx \]

Now, integrate both sides of the equation:
\[ \int \frac{1}{1+y^2} \,dy = \int 1 \,dx \]

Performing the integration gives:
\[ \tan^{-1}(y) = x + c \]

where \(c\) is the constant of integration.

To find the explicit solution for \(y\), we take the tangent of both sides:
\[ y = \tan(x+c) \]


Step 4: Final Answer:

The solution of the differential equation is \(y = \tan(x+c)\).
Quick Tip: When solving differential equations, remember that the constant of integration \(c\) is added immediately after integrating. In this case, \[ \tan^{-1}(y) = x + c \] becomes \[ y = \tan(x + c), \] which is different from \[ y = \tan(x) + c. \] The position of the constant is crucial.

Step 1: Understanding the Question:

We need to solve a first-order linear differential equation with a given initial condition (an Initial Value Problem).


Step 2: Key Formula or Approach:

The equation is in the linear form \(\frac{dy}{dx} + P(x)y = Q(x)\). We solve it using an integrating factor (I.F.).

1. Identify \(P(x)\) and \(Q(x)\).

2. Calculate the Integrating Factor: I.F. = \(e^{\int P(x)dx}\).

3. The general solution is given by: \(y \cdot (I.F.) = \int Q(x) \cdot (I.F.) \,dx + C\).

4. Use the initial condition \(y(1)=1\) to find the value of the constant \(C\).


Step 3: Detailed Explanation:

The given equation is \(\frac{dy}{dx} + \frac{1}{x}y = x^2\).

1. Here, \(P(x) = \frac{1}{x}\) and \(Q(x) = x^2\).

2. Calculate the integrating factor:
\[ I.F. = e^{\int \frac{1}{x}dx} = e^{\ln x} = x \quad (for x>0) \]

3. Find the general solution:
\[ y \cdot x = \int x^2 \cdot x \,dx + C \]
\[ xy = \int x^3 \,dx + C \]
\[ xy = \frac{x^4}{4} + C \]

4. Apply the initial condition \(y(1) = 1\) (when \(x=1\), \(y=1\)):
\[ (1)(1) = \frac{(1)^4}{4} + C \]
\[ 1 = \frac{1}{4} + C \]
\[ C = 1 - \frac{1}{4} = \frac{3}{4} \]

Substitute the value of \(C\) back into the general solution:
\[ xy = \frac{x^4}{4} + \frac{3}{4} \]

To match the format of the options, multiply the entire equation by 4:
\[ 4xy = x^4 + 3 \]


Step 4: Final Answer:

The solution of the initial value problem is \(4xy = x^4 + 3\).
Quick Tip: The integrating factor method is a standard procedure for first-order linear DEs. Remember the three key steps: find P(x) and Q(x), calculate the I.F., and then apply the solution formula \(y \cdot (I.F.) = \int Q(x) \cdot (I.F.) \,dx + C\).

Step 1: Understanding the Question:

We need to find the general solution for a third-order homogeneous linear differential equation with constant coefficients.


Step 2: Key Formula or Approach:

We solve this by finding the roots of the auxiliary (or characteristic) equation.

1. Form the auxiliary equation by replacing \(\frac{d^ny}{dx^n}\) with \(m^n\).

2. Find the roots of the resulting polynomial equation.

3. The form of the general solution depends on the nature of these roots (real and distinct, real and repeated, or complex).


Step 3: Detailed Explanation:

The given differential equation is \(y''' + 3y'' + 2y' = 0\).

1. The auxiliary equation is:
\[ m^3 + 3m^2 + 2m = 0 \]

2. Factor the polynomial to find the roots:
\[ m(m^2 + 3m + 2) = 0 \]

Factor the quadratic part:
\[ m(m+1)(m+2) = 0 \]

The roots are \(m_1 = 0\), \(m_2 = -1\), and \(m_3 = -2\).

3. Since we have three distinct real roots, the general solution is of the form:
\[ y = c_1 e^{m_1 x} + c_2 e^{m_2 x} + c_3 e^{m_3 x} \]

Substituting the roots we found:
\[ y = c_1 e^{0x} + c_2 e^{-1x} + c_3 e^{-2x} \]

Since \(e^{0x} = 1\), the solution is:
\[ y = c_1(1) + c_2 e^{-x} + c_3 e^{-2x} \]

Using the arbitrary constants \(a, b, c\) from the options, this becomes:
\[ y = a + be^{-x} + ce^{-2x} \]


Step 4: Final Answer:

The general solution is \(y = a + be^{-x} + ce^{-2x}\).
Quick Tip: For homogeneous linear DEs with constant coefficients, the process is always the same: form the auxiliary equation, find its roots, and write the solution based on the type of roots. A root of \(m=0\) always contributes a simple constant term to the solution.

Step 1: Understanding the Question:

We need to find the particular integral (PI) for a second-order non-homogeneous linear differential equation with constant coefficients.


Step 2: Key Formula or Approach:

We use the operator method. The particular integral \(y_p\) is given by \(y_p = \frac{1}{f(D)} R(x)\), where \(D = \frac{d}{dx}\), \(f(D)\) is the differential operator, and \(R(x)\) is the function on the right-hand side.

For \(R(x) = e^{ax}\), we evaluate \(\frac{1}{f(a)}e^{ax}\). If \(f(a)=0\) (case of failure), the rule is \(y_p = x \frac{1}{f'(a)}e^{ax}\).


Step 3: Detailed Explanation:

The differential equation is \((D^2 + 3D + 2)y = e^{-2x}\).

So, \(f(D) = D^2 + 3D + 2\) and \(R(x) = e^{-2x}\).

The particular integral is:
\[ y_p = \frac{1}{D^2 + 3D + 2} e^{-2x} \]

We have the form \(e^{ax}\) with \(a=-2\). Let's evaluate \(f(a) = f(-2)\):
\[ f(-2) = (-2)^2 + 3(-2) + 2 = 4 - 6 + 2 = 0 \]

This is a "case of failure" because the denominator becomes zero. This happens because \(a=-2\) is a root of the auxiliary equation \(m^2+3m+2=0\).

When failure occurs, we apply the rule: multiply by \(x\) and differentiate the denominator with respect to \(D\).
\[ f'(D) = \frac{d}{dD}(D^2 + 3D + 2) = 2D + 3 \]

Now, the particular integral is given by:
\[ y_p = x \frac{1}{f'(D)} e^{-2x} = x \frac{1}{2D + 3} e^{-2x} \]

Substitute \(D=-2\) into the new operator:
\[ y_p = x \frac{1}{2(-2) + 3} e^{-2x} = x \frac{1}{-4 + 3} e^{-2x} = x \frac{1}{-1} e^{-2x} \]
\[ y_p = -xe^{-2x} \]


Step 4: Final Answer:

The particular integral of the differential equation is \(-xe^{-2x}\).
Quick Tip: When finding the particular integral for \(e^{ax}\) using the operator method, always first check if 'a' is a root of the auxiliary equation. If it is (i.e., if \(f(a)=0\)), you know it's a case of failure and you must apply the rule of multiplying by x and differentiating the denominator.

Step 1: Understanding the Question:

We are asked to find the dimensional formula for Power (P) using Force (F), Velocity (V), and Length (L) as the fundamental units instead of Mass (M), Length (L), and Time (T).


Step 2: Key Formula or Approach:

We will use the method of dimensional analysis.

1. Write the dimensions of Power and the new fundamental quantities in terms of the standard M, L, T system.

- Power \([P] = [ML^2T^{-3}]\)

- Force \([F] = [MLT^{-2}]\)

- Velocity \([V] = [LT^{-1}]\)

- Length \([L] = [L]\)

2. Assume that Power is related to F, V, and L by the equation \(P = k F^a V^b L^c\), where k is a dimensionless constant and a, b, c are the powers we need to find.

3. Equate the dimensions on both sides and solve for a, b, and c.


Step 3: Detailed Explanation:

Set up the dimensional equation:
\[ [P] = [F]^a [V]^b [L]^c \]

Substitute the standard dimensions:
\[ [ML^2T^{-3}] = [MLT^{-2}]^a [LT^{-1}]^b [L]^c \]
\[ [M^1L^2T^{-3}] = [M^a L^a T^{-2a}] [L^b T^{-b}] [L^c] \]

Combine the powers on the right side:
\[ [M^1L^2T^{-3}] = [M^a L^{a+b+c} T^{-2a-b}] \]

Now, equate the powers of M, L, and T from both sides:

- For M: \(a = 1\)

- For T: \(-2a - b = -3\)

- For L: \(a + b + c = 2\)

Solve the system of equations:

From the M equation, we have \(a = 1\).

Substitute \(a=1\) into the T equation:
\[ -2(1) - b = -3 \implies -2 - b = -3 \implies b = 1 \]

Substitute \(a=1\) and \(b=1\) into the L equation:
\[ 1 + 1 + c = 2 \implies 2 + c = 2 \implies c = 0 \]

So, the powers are \(a=1, b=1, c=0\). The expression for Power is \(F^1 V^1 L^0\).


Step 4: Final Answer:

Power can be expressed as \(F^1 L^0 V^1\).
Quick Tip: A much faster method is to use known physical relationships. We know that Power is the dot product of Force and Velocity: \(P = \vec{F} \cdot \vec{v}\). Dimensionally, this is simply \([P] = [F][V]\). This directly gives the answer as \(F^1 V^1 L^0\).

Step 1: Understanding the Question:

We need to check the dimensional formulas for each pair of physical quantities listed in the options and find the pair with identical dimensions.


Step 2: Key Formula or Approach:

We will derive the dimensional formula for each quantity based on its physical definition or formula. The fundamental dimensions are Mass (M), Length (L), and Time (T).


Step 3: Detailed Explanation:

Let's analyze each option:

(A) Torque and momentum:

- Torque (\(\tau\)) = Force \(\times\) perpendicular distance = \([MLT^{-2}] \times [L] = [ML^2T^{-2}]\).

- Momentum (\(p\)) = mass \(\times\) velocity = \([M] \times [LT^{-1}] = [MLT^{-1}]\).

The dimensions are not the same.


(B) Surface tension and tension:

- Surface Tension = Force per unit length = \([MLT^{-2}] / [L] = [MT^{-2}]\).

- Tension is a type of force, so its dimension is \([MLT^{-2}]\).

The dimensions are not the same.


(C) Pressure and modulus of elasticity:

- Pressure (\(P\)) = Force / Area = \([MLT^{-2}] / [L^2] = [ML^{-1}T^{-2}]\).

- Modulus of Elasticity (\(E\)) = Stress / Strain.

- Stress = Force / Area = \([MLT^{-2}] / [L^2] = [ML^{-1}T^{-2}]\).

- Strain = Change in dimension / Original dimension = \([L]/[L] = [M^0L^0T^0]\) (dimensionless).

- Therefore, the dimension of Modulus of Elasticity is the same as Stress: \([ML^{-1}T^{-2}]\).

The dimensions of Pressure and Modulus of Elasticity are the same.


(D) Force constant and Planck's constant:

- Force constant (\(k\)) from Hooke's Law (F=kx) = Force / distance = \([MLT^{-2}] / [L] = [MT^{-2}]\).

- Planck's constant (\(h\)) from (E=h\(\nu\)) = Energy / frequency = \([ML^2T^{-2}] / [T^{-1}] = [ML^2T^{-1}]\).

The dimensions are not the same.


Step 4: Final Answer:

The pair with the same dimensional formula is Pressure and modulus of elasticity.
Quick Tip: Remember that quantities like Pressure, Stress, and any Modulus of Elasticity (Young's, Bulk, Shear) all share the same dimension of Force/Area, which is \([ML^{-1}T^{-2}]\). This is because strain is always dimensionless.

Step 1: Understanding the Question:

We are given a vector relationship \(\vec{A} + \vec{B} = \vec{C}\) and a scalar relationship between their magnitudes, \(A^2 + B^2 = C^2\). We need to find the angle \(\theta\) between vectors \(\vec{A}\) and \(\vec{B}\).


Step 2: Key Formula or Approach:

The magnitude of the resultant vector \(\vec{C} = \vec{A} + \vec{B}\) is given by the law of cosines for vectors:
\[ C = |\vec{C}| = \sqrt{A^2 + B^2 + 2AB \cos\theta} \]

Squaring both sides gives:
\[ C^2 = A^2 + B^2 + 2AB \cos\theta \]


Step 3: Detailed Explanation:

We have two expressions for \(C^2\):

1. From the magnitude of the vector sum: \(C^2 = A^2 + B^2 + 2AB \cos\theta\)

2. From the given information: \(C^2 = A^2 + B^2\)

Equating these two expressions for \(C^2\):
\[ A^2 + B^2 = A^2 + B^2 + 2AB \cos\theta \]

Subtract \(A^2 + B^2\) from both sides:
\[ 0 = 2AB \cos\theta \]

Assuming the vectors \(\vec{A}\) and \(\vec{B}\) are non-zero vectors (so their magnitudes \(A\) and \(B\) are non-zero), the only way for the product to be zero is if \(\cos\theta = 0\).
\[ \cos\theta = 0 \]

The angle \(\theta\) for which \(\cos\theta = 0\) is \(90^{\circ}\) or \(\frac{\pi}{2}\) radians.


Step 4: Final Answer:

The angle between vectors \(\vec{A}\) and \(\vec{B}\) is \(90^{\circ}\).
Quick Tip: The condition \(A^2 + B^2 = C^2\) for the sum \(\vec{C} = \vec{A} + \vec{B}\) is the vector equivalent of the Pythagorean theorem. It holds true only when the vectors \(\vec{A}\) and \(\vec{B}\) are perpendicular to each other.

Step 1: Understanding the Question:

We are asked to find the area of a rectangle. The vectors \(\vec{A}\) and \(\vec{B}\) are given to represent the sides. The area of a rectangle is the product of the lengths of its adjacent sides. The wording implies that the lengths of the sides of the rectangle are given by the magnitudes of the vectors \(\vec{A}\) and \(\vec{B}\).


Step 2: Key Formula or Approach:

Area of a rectangle = length \(\times\) width.

The length of a vector \(\vec{V} = x\hat{i} + y\hat{j}\) is its magnitude, given by \(|\vec{V}| = \sqrt{x^2 + y^2}\).


Step 3: Detailed Explanation:

First, we find the length of the side represented by vector \(\vec{A}\).
\[ |\vec{A}| = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5 \]

Next, we find the length of the side represented by vector \(\vec{B}\).
\[ |\vec{B}| = \sqrt{1^2 + 3^2} = \sqrt{1 + 9} = \sqrt{10} \]

Now, we calculate the area of the rectangle:
\[ Area = |\vec{A}| \times |\vec{B}| = 5 \times \sqrt{10} = 5\sqrt{10} units \]

Note: For the vectors to represent the sides of a rectangle, they should be perpendicular (\(\vec{A} \cdot \vec{B} = 0\)). Let's check: \(\vec{A} \cdot \vec{B} = (3)(1) + (4)(3) = 15 \neq 0\). The vectors are not perpendicular. The question is poorly phrased, but the intended meaning is to use the magnitudes of the given vectors as the lengths of the rectangle's sides.


Step 4: Final Answer:

The area of the rectangle is \(5\sqrt{10}\) units.
Quick Tip: When a question about a geometric shape provides vectors for its sides, it usually implies that the magnitudes of those vectors should be used as the lengths. If the shape was a parallelogram, the area would be given by the magnitude of the cross product, \(|\vec{A} \times \vec{B}|\).

Step 1: Understanding the Question:

We are given the initial upward velocity of a pebble thrown from a tower, the total time of flight, and the acceleration due to gravity. We need to find the final velocity just before it hits the ground.


Step 2: Key Formula or Approach:

We can use the first equation of motion for an object under constant acceleration:
\[ v = u + at \]

where \(v\) is the final velocity, \(u\) is the initial velocity, \(a\) is the acceleration, and \(t\) is the time.


Step 3: Detailed Explanation:

Let's establish a sign convention. We will consider the upward direction as positive and the downward direction as negative.

- Initial velocity, \(u = +5\) m/s (since it's thrown upwards).

- Acceleration, \(a = -g = -10\) m/s\(^2\) (gravity acts downwards).

- Time of flight, \(t = 3\) s.

Now, substitute these values into the equation of motion:
\[ v = 5 + (-10)(3) \]
\[ v = 5 - 30 \]
\[ v = -25 m/s \]

The negative sign indicates that the final velocity is in the downward direction. The question asks for the velocity with which it strikes, and the options are all positive, implying we need to find the speed.

The speed of striking the ground is \(|v| = 25\) m/s.


Step 4: Final Answer:

The pebble strikes the ground with a velocity of 25 m/s.
Quick Tip: For projectile motion problems, consistently applying a sign convention is crucial. Choosing 'up' as positive and 'down' as negative is a standard convention that helps avoid confusion with the signs of velocity and acceleration.

Step 1: Understanding the Question:

A body falls from rest from a height H, taking time T. We need to find its height from the ground at time T/2.


Step 2: Key Formula or Approach:

We use the equation of motion for distance traveled under constant acceleration, starting from rest:
\[ s = ut + \frac{1}{2}at^2 \]

Since the body is released from rest, \(u=0\). Let's take the downward direction as positive, so \(a=g\). The distance fallen from the top is \(s = \frac{1}{2}gt^2\).


Step 3: Detailed Explanation:

First, relate the total height H to the total time T. In time T, the body falls a distance H.
\[ H = \frac{1}{2}gT^2 \quad (Equation 1) \]

Next, find the distance the body has fallen from the top at time \(t = T/2\). Let's call this distance \(s_{T/2}\).
\[ s_{T/2} = \frac{1}{2}g\left(\frac{T}{2}\right)^2 = \frac{1}{2}g\frac{T^2}{4} = \frac{1}{4} \left(\frac{1}{2}gT^2\right) \]

From Equation 1, we know that \(\frac{1}{2}gT^2 = H\). So, we can substitute H into the expression for \(s_{T/2}\):
\[ s_{T/2} = \frac{H}{4} \]

This is the distance fallen from the top of the tower. The question asks for the position (height) of the body from the ground.
\[ Height from ground = Total Height - Distance fallen \]
\[ Height from ground = H - s_{T/2 = H - \frac{H}{4} = \frac{3H}{4} \]


Step 4: Final Answer:

At time T/2, the body is at a height of \(\frac{3H}{4}\) from the ground.
Quick Tip: For an object in free fall from rest, the distance covered is proportional to the square of the time (\(s \propto t^2\)). This means in half the total time, it covers \((1/2)^2 = 1/4\) of the total distance. Therefore, the remaining distance to the ground is \(1 - 1/4 = 3/4\) of the total height.

Step 1: Understanding the Question:

We are given that a body starts from rest with constant acceleration. We need to find the relationship between the distance covered in the first 2 seconds and the distance covered in the subsequent 2 seconds.


Step 2: Key Formula or Approach:

We use the equation of motion for displacement: \(s = ut + \frac{1}{2}at^2\).

Since the body starts from rest, the initial velocity \(u=0\). The formula simplifies to \(s = \frac{1}{2}at^2\).


Step 3: Detailed Explanation:

Let the uniform acceleration be \(a\).

The distance covered in the first 2 seconds (\(t_1 = 2\) s) is \(x\).
\[ x = \frac{1}{2}a(t_1)^2 = \frac{1}{2}a(2)^2 = \frac{1}{2}a(4) = 2a \]

The distance covered in the "next 2 seconds" means the distance traveled between \(t=2\) s and \(t=4\) s. This can be found by calculating the total distance in 4 seconds and subtracting the distance covered in the first 2 seconds.

Total time for both intervals is \(t_2 = 4\) s.

Total distance covered in 4 seconds is \(s_{total}\).
\[ s_{total} = \frac{1}{2}a(t_2)^2 = \frac{1}{2}a(4)^2 = \frac{1}{2}a(16) = 8a \]

The distance covered in the next 2 seconds, \(y\), is:
\[ y = s_{total} - x = 8a - 2a = 6a \]

Now, we find the relationship between \(y\) and \(x\):

We have \(x = 2a\) and \(y = 6a\).
\[ y = 6a = 3 \times (2a) = 3x \]


Step 4: Final Answer:

The relationship between y and x is \(y = 3x\).
Quick Tip: According to Galileo's law of odd numbers, the distances traversed during equal intervals of time by a body falling from rest stand to one another in the same ratio as the odd numbers beginning with unity (1:3:5:7...). Since the time intervals are equal (2s each), the ratio of distances \(x:y\) will be 1:3, which means \(y=3x\).

Step 1: Understanding the Question:

A juggler throws \(n\) balls per second. The time interval between throws is the time it takes for a ball to reach its maximum height. We need to find this maximum height.


Step 2: Key Formula or Approach:

1. Determine the time of flight to the highest point.

2. Use kinematic equations to relate this time to the initial velocity (\(v=u+at\)).

3. Use another kinematic equation to relate the initial velocity to the maximum height (\(v^2=u^2+2as\)).


Step 3: Detailed Explanation:

If the juggler throws \(n\) balls each second, the time interval between two consecutive throws is \(\Delta t = \frac{1}{n}\) seconds.

The problem states this is the time for a ball to reach its highest point. Let's call this time \(t_{up}\).
\[ t_{up} = \frac{1}{n} \]

At the maximum height, the final vertical velocity \(v\) is 0. Using \(v = u + at\) with \(a = -g\) (upwards as positive):
\[ 0 = u - g \cdot t_{up} \]
\[ u = g \cdot t_{up} = g \cdot \frac{1}{n} = \frac{g}{n} \]

This is the initial velocity with which each ball is thrown.

Now, to find the maximum height \(H\), we use the equation \(v^2 = u^2 + 2as\):
\[ 0^2 = u^2 + 2(-g)H \]
\[ u^2 = 2gH \]
\[ H = \frac{u^2}{2g} \]

Substitute the expression for \(u\) we found:
\[ H = \frac{(g/n)^2}{2g} = \frac{g^2/n^2}{2g} = \frac{g}{2n^2} \]


Step 4: Final Answer:

The balls rise to a height of \(\frac{g}{2n^2}\).
Quick Tip: This problem connects rate (\(n\) balls per second) to time (\(t = 1/n\)). Once the time to reach the peak is known, the kinematics of the projectile can be fully determined. Breaking down the problem statement into physical quantities is the first crucial step.

Step 1: Understanding the Question:

We need to find the minimum force required to push a block up an inclined plane, overcoming both gravity and friction.


Step 2: Key Formula or Approach:

We will use a free-body diagram and apply Newton's First Law (for the condition of impending motion, acceleration is zero). The main forces are the applied force, gravity, normal force, and friction.


Step 3: Detailed Explanation:

Let's analyze the forces acting on the block along axes parallel and perpendicular to the inclined plane.

1. Gravitational Force (Weight): \(mg\), acting vertically downwards.
- Component parallel to the incline: \(mg \sin \theta\) (acting down the incline).
- Component perpendicular to the incline: \(mg \cos \theta\) (acting into the incline).

2. Normal Force (N): Acts perpendicular to the surface, outwards. From equilibrium in the perpendicular direction, \(N = mg \cos \theta\).

3. Frictional Force (f): Opposes the motion (or impending motion) up the plane, so it acts down the plane. The maximum static friction (or kinetic friction) is \(f = \mu N = \mu mg \cos \theta\).

4. Applied Force (F): The force required to move the block up the plane, acting parallel to the incline, upwards.

For the block to move up, the applied force F must overcome the sum of the forces pulling it down the incline.
\[ F = (Gravitational component down the incline) + (Frictional force down the incline) \]
\[ F = mg \sin \theta + f \]

Substitute \(f = \mu mg \cos \theta\):
\[ F = mg \sin \theta + \mu mg \cos \theta \]


Step 4: Final Answer:

The force required to move the block up the inclined plane is \(mg \sin \theta + \mu mg \cos \theta\).
Quick Tip: When an object is pushed \textbf{up} an incline, both the parallel component of gravity (\(mg \sin \theta\)) and friction (\(\mu mg \cos \theta\)) act in the same direction (down the incline) and must be overcome. Thus, they add up. If the object were sliding \textbf{down}, friction would act up the incline, opposing the gravitational component.

Step 1: Understanding the Question:

A body starts from rest on a smooth (frictionless) incline. Given the time for the full journey, we need to find the time it takes to cover the first quarter of the distance.


Step 2: Key Formula or Approach:

For an object starting from rest (\(u=0\)) and moving with constant acceleration (\(a\)), the distance covered (\(s\)) in time (\(t\)) is given by the kinematic equation:
\[ s = \frac{1}{2}at^2 \]

From this, we can see that the distance is proportional to the square of the time (\(s \propto t^2\)).


Step 3: Detailed Explanation:

Let \(L\) be the total length of the inclined plane and \(T = 4\) s be the total time to slide down.

Let \(t\) be the time taken to slide a distance of \(s = L/4\).

Using the proportionality \(s \propto t^2\), we can set up a ratio:
\[ \frac{s_1}{s_2} = \frac{t_1^2}{t_2^2} \]

Let \(s_1 = L\), \(t_1 = T = 4\) s.

Let \(s_2 = L/4\), \(t_2 = t\).
\[ \frac{L}{L/4} = \frac{4^2}{t^2} \]
\[ 4 = \frac{16}{t^2} \]

Rearrange to solve for \(t^2\):
\[ t^2 = \frac{16}{4} = 4 \]
\[ t = \sqrt{4} = 2 s \]


Step 4: Final Answer:

The time taken to slide 1/4th of the length is 2 seconds.
Quick Tip: The relationship \(s \propto t^2\) for motion from rest is very powerful. It implies that \(t \propto \sqrt{s}\). To cover 1/4 of the distance, it will take \(\sqrt{1/4} = 1/2\) of the total time. Half of the total time of 4 seconds is 2 seconds.

Step 1: Understanding the Question:

We are given the mass and the initial and final velocity vectors of a body. We need to calculate the change in its kinetic energy.


Step 2: Key Formula or Approach:

The change in kinetic energy (\(\Delta KE\)) is the final kinetic energy minus the initial kinetic energy.
\[ \Delta KE = KE_{final} - KE_{initial} \]

The kinetic energy is given by \(KE = \frac{1}{2}mv^2\), where \(v\) is the speed (magnitude of the velocity vector).


Step 3: Detailed Explanation:

The mass of the body is \(m = 2\) Kg.

Initial velocity, \(\vec{v}_i = 3\hat{i} - 4\hat{j}\).

Final velocity, \(\vec{v}_f = 6\hat{j} + 2\hat{k}\).


First, calculate the initial speed squared (\(v_i^2\)):
\[ v_i^2 = |\vec{v}_i|^2 = (3)^2 + (-4)^2 = 9 + 16 = 25 \, (m/s)^2 \]

Now, calculate the initial kinetic energy (\(KE_i\)):
\[ KE_i = \frac{1}{2}mv_i^2 = \frac{1}{2}(2)(25) = 25 J \]


Next, calculate the final speed squared (\(v_f^2\)):
\[ v_f^2 = |\vec{v}_f|^2 = (6)^2 + (2)^2 = 36 + 4 = 40 \, (m/s)^2 \]

Now, calculate the final kinetic energy (\(KE_f\)):
\[ KE_f = \frac{1}{2}mv_f^2 = \frac{1}{2}(2)(40) = 40 J \]


Finally, calculate the change in kinetic energy:
\[ \Delta KE = KE_f - KE_i = 40 J - 25 J = 15 J \]


Step 4: Final Answer:

The change in kinetic energy of the body is 15 J.
Quick Tip: According to the Work-Energy Theorem, the net work done on an object equals its change in kinetic energy. This calculation gives the net work done on the body to change its velocity. Remember to find the magnitude (speed) from the velocity vector before calculating KE.

Step 1: Understanding the Question:

We need to find the maximum velocity of a girl on a swing, given her heights at the highest and lowest points of her swing. The maximum velocity occurs at the lowest point.


Step 2: Key Formula or Approach:

We will use the principle of conservation of mechanical energy. The total mechanical energy (Kinetic Energy + Potential Energy) at the highest point is equal to the total mechanical energy at the lowest point, assuming no air resistance.
\[ KE_{top} + PE_{top} = KE_{bottom} + PE_{bottom} \]
\[ \frac{1}{2}mv_{top}^2 + mgh_{top} = \frac{1}{2}mv_{bottom}^2 + mgh_{bottom} \]


Step 3: Detailed Explanation:

Let's define the given values:

- Height at the top, \(h_{top} = 3\) m.

- Height at the bottom, \(h_{bottom} = 2\) m.

- At the maximum height (top), the swing momentarily stops, so \(v_{top} = 0\).

- At the lowest point (bottom), the velocity is maximum, so \(v_{bottom} = v_{max}\).

- We use the standard value for acceleration due to gravity, \(g = 9.8\) m/s\(^2\).

Substitute these into the conservation of energy equation:
\[ \frac{1}{2}m(0)^2 + mgh_{top} = \frac{1}{2}mv_{max}^2 + mgh_{bottom} \]
\[ 0 + mgh_{top} = \frac{1}{2}mv_{max}^2 + mgh_{bottom} \]

The mass \(m\) cancels from all terms:
\[ gh_{top} = \frac{1}{2}v_{max}^2 + gh_{bottom} \]

Rearrange to solve for \(v_{max}^2\):
\[ \frac{1}{2}v_{max}^2 = gh_{top} - gh_{bottom} = g(h_{top} - h_{bottom}) \]
\[ v_{max}^2 = 2g(h_{top} - h_{bottom}) \]

Substitute the numerical values:
\[ v_{max}^2 = 2(9.8)(3 - 2) = 2(9.8)(1) = 19.6 \]
\[ v_{max} = \sqrt{19.6} m/s \]


Step 4: Final Answer:

Her maximum velocity is \(\sqrt{19.6}\) m/s.
Quick Tip: In energy conservation problems, the change in kinetic energy is equal to the negative of the change in potential energy: \(\Delta KE = -\Delta PE\). Here, the loss in potential energy, \(mg(h_{top} - h_{bottom})\), is converted into a gain in kinetic energy, \(\frac{1}{2}mv_{max}^2\).

Step 1: Understanding the Question:

We are given the output power of an engine and its efficiency. We need to calculate the power that is supplied to the engine, which is the input power.


Step 2: Key Formula or Approach:

The efficiency (\(\eta\)) of an engine is defined as the ratio of the useful output power to the total input power, usually expressed as a percentage.
\[ \eta = \frac{Output Power}{Input Power} \]

To find the input power, we can rearrange this formula:
\[ Input Power = \frac{Output Power}{\eta} \]


Step 3: Detailed Explanation:

Given values are:

- Output Power = 1000 W

- Efficiency, \(\eta = 80% = \frac{80}{100} = 0.8\)

Using the rearranged formula to find the Input Power:
\[ Input Power = \frac{1000 W}{0.8} \]
\[ Input Power = \frac{10000}{8} W \]
\[ Input Power = 1250 W \]


Step 4: Final Answer:

The input power required for the engine is 1250 W.
Quick Tip: Remember that efficiency is always less than 1 (or 100%). Therefore, the input power must always be greater than the output power, as some energy is always lost (usually as heat). This can help you eliminate options like 800 W and 1000 W immediately.

Step 1: Understanding the Question:

We are considering a "seconds pendulum," which has a specific time period on Earth. We need to find its new time period on a planet with different gravity.


Step 2: Key Formula or Approach:

A seconds pendulum is defined as a pendulum having a time period of exactly 2 seconds on Earth.

The formula for the time period (\(T\)) of a simple pendulum is:
\[ T = 2\pi\sqrt{\frac{L}{g}} \]

where \(L\) is the length of the pendulum and \(g\) is the acceleration due to gravity. From this formula, we can see that the time period is inversely proportional to the square root of gravity (\(T \propto \frac{1}{\sqrt{g}}\)).


Step 3: Detailed Explanation:

Let \(T_E\) and \(g_E\) be the time period and gravity on Earth.

Let \(T_P\) and \(g_P\) be the time period and gravity on the planet.

We are given:

- \(T_E = 2\) s (definition of a seconds pendulum).

- \(g_P = \frac{g_E}{2}\).

Using the proportionality \(T \propto \frac{1}{\sqrt{g}}\), we can set up a ratio:
\[ \frac{T_P}{T_E} = \frac{1/\sqrt{g_P}}{1/\sqrt{g_E}} = \sqrt{\frac{g_E}{g_P}} \]

Substitute the given relationship for gravity:
\[ \frac{T_P}{T_E} = \sqrt{\frac{g_E}{g_E/2}} = \sqrt{2} \]

Now, solve for the time period on the planet, \(T_P\):
\[ T_P = T_E \times \sqrt{2} \]
\[ T_P = 2 \times \sqrt{2} = 2\sqrt{2} sec \]


Step 4: Final Answer:

The time period of the pendulum on the planet is \(2\sqrt{2}\) seconds.
Quick Tip: Remember the definition of a "seconds pendulum" (its period is 2s, not 1s, because one "tick" or half-period is 1s). Also, recall the inverse square root relationship between period and gravity. Lower gravity means a longer (slower) period.

Step 1: Understanding the Question:

We need to find the position (\(x\)) of a particle in Simple Harmonic Motion (SHM) when its velocity (\(v\)) is half of its maximum possible velocity (\(v_{max}\)).


Step 2: Key Formula or Approach:

The velocity of a particle in SHM as a function of its position \(x\) is given by:
\[ v = \omega \sqrt{A^2 - x^2} \]

where \(\omega\) is the angular frequency and \(A\) is the amplitude.

The maximum velocity occurs at the equilibrium position (\(x=0\)) and is given by:
\[ v_{max} = A\omega \]


Step 3: Detailed Explanation:

We are given the condition that \(v = \frac{1}{2}v_{max}\).

Substitute the formulas for \(v\) and \(v_{max}\) into this condition:
\[ \omega \sqrt{A^2 - x^2} = \frac{1}{2}(A\omega) \]

The angular frequency \(\omega\) cancels from both sides:
\[ \sqrt{A^2 - x^2} = \frac{A}{2} \]

To solve for \(x\), square both sides of the equation:
\[ A^2 - x^2 = \left(\frac{A}{2}\right)^2 = \frac{A^2}{4} \]

Now, isolate \(x^2\):
\[ x^2 = A^2 - \frac{A^2}{4} = \frac{4A^2 - A^2}{4} = \frac{3A^2}{4} \]

Take the square root of both sides to find the position \(x\):
\[ x = \pm \sqrt{\frac{3A^2}{4}} = \pm \frac{\sqrt{3}A}{2} \]

The question asks for the position, and the positive value is given in the options.


Step 4: Final Answer:

The position of the particle is at \(\frac{\sqrt{3}A}{2}\).
Quick Tip: The key relationship \(v = \omega \sqrt{A^2 - x^2}\) is fundamental to SHM. It's derived from the conservation of energy in the oscillator system. Memorizing this formula is essential for solving problems that relate position and velocity in SHM.

Step 1: Understanding the Question:

We are given an equation for the displacement of a particle which is a sum of a sine and a cosine function with the same frequency. We need to find the amplitude of the resulting Simple Harmonic Motion (SHM).


Step 2: Key Formula or Approach:

An expression of the form \(x = a \sin(\omega t) + b \cos(\omega t)\) represents an SHM. The amplitude \(A\) of this resultant motion is given by:
\[ A = \sqrt{a^2 + b^2} \]

The expression can be rewritten as \(x = A \sin(\omega t + \phi)\) or \(x = A \cos(\omega t + \delta)\).


Step 3: Detailed Explanation:

The given equation for displacement is:
\[ x = 3 \sin(2t) + 4 \cos(2t) \]

This matches the standard form \(x = a \sin(\omega t) + b \cos(\omega t)\) with:

- \(a = 3\)

- \(b = 4\)

- \(\omega = 2\) rad/s

Now, we can calculate the resultant amplitude \(A\) using the formula:
\[ A = \sqrt{a^2 + b^2} = \sqrt{3^2 + 4^2} \]
\[ A = \sqrt{9 + 16} = \sqrt{25} \]
\[ A = 5 \]

The unit of the amplitude would be the same as the unit of displacement \(x\).


Step 4: Final Answer:

The amplitude of the particle is 5.
Quick Tip: This is a direct application of the superposition of two perpendicular vectors (or phasors in this context). If you have a vector with components 3 and 4, its magnitude is 5. This is a classic 3-4-5 Pythagorean triple, which often appears in physics problems. Recognizing it can provide an instant answer.

Step 1: Understanding the Question:

We are comparing the maximum intensity during the phenomenon of beats with the intensity of a single sound source. The sources have equal amplitudes.


Step 2: Key Formula or Approach:

The intensity (\(I\)) of a wave is proportional to the square of its amplitude (\(A\)).
\[ I \propto A^2 \]

When two waves interfere, the resultant amplitude depends on the phase difference. For beats, the waves cyclically go in and out of phase.

- At constructive interference (maximum loudness), the amplitudes add up.

- At destructive interference (minimum loudness), the amplitudes subtract.


Step 3: Detailed Explanation:

Let the amplitude of each individual sound source be \(A_0\).

The intensity of a single source, \(I_0\), is proportional to \(A_0^2\).
\[ I_0 = k A_0^2 \] (where k is a proportionality constant)

During the formation of beats, the maximum intensity occurs at points of constructive interference. At these points, the amplitudes of the two waves add.

The maximum resultant amplitude, \(A_{max}\), is:
\[ A_{max} = A_0 + A_0 = 2A_0 \]

The maximum intensity, \(I_{max}\), is proportional to the square of this maximum amplitude:
\[ I_{max} = k (A_{max})^2 = k (2A_0)^2 = k (4A_0^2) \]

Now, let's compare the maximum intensity \(I_{max}\) with the intensity of one source \(I_0\):
\[ I_{max} = 4 (k A_0^2) = 4 I_0 \]

This means the maximum intensity is four times the intensity of a single source.


Step 4: Final Answer:

The maximum intensity of beats will be four times the intensity of one source.
Quick Tip: A common mistake is to think that if amplitude doubles, intensity also doubles. Remember that intensity is proportional to the \textbf{square} of the amplitude. So, if amplitude becomes \(n\) times, intensity becomes \(n^2\) times. Here, amplitude doubles (from \(A_0\) to \(2A_0\)), so intensity becomes \(2^2 = 4\) times.

Step 1: Understanding the Question:

This is a problem on the Doppler effect for sound waves. The sound source is moving away from a stationary listener, and we need to find the apparent frequency heard by the listener.


Step 2: Key Formula or Approach:

The general formula for the Doppler effect is:
\[ f' = f \left( \frac{v \pm v_L}{v \mp v_S} \right) \]

where \(f'\) is the apparent frequency, \(f\) is the source frequency, \(v\) is the speed of sound, \(v_L\) is the speed of the listener, and \(v_S\) is the speed of the source.

In our case:

- The listener is static, so \(v_L = 0\).

- The source is moving away from the listener. This should cause the apparent frequency to decrease. To make the fraction smaller, we use a '+' sign in the denominator.

So the formula becomes:
\[ f' = f \left( \frac{v}{v + v_S} \right) \]


Step 3: Detailed Explanation:

We are given the following values:

- Source frequency, \(f = 800\) Hz.

- Speed of sound, \(v = 340\) m/s.

- Speed of the source, \(v_S = 30\) m/s.

Substitute these values into the formula:
\[ f' = 800 \left( \frac{340}{340 + 30} \right) \]
\[ f' = 800 \left( \frac{340}{370} \right) \]
\[ f' = 800 \times \frac{34}{37} \]
\[ f' = \frac{27200}{37} \approx 735.13 Hz \]

This value is closest to 733.3 Hz. The small discrepancy might be due to rounding in the problem's intended answer or a slightly different value for the speed of sound used. 733.3 Hz is the most plausible answer.

Let's re-calculate with 733.3 Hz as the target. \(800 * (340/370) = 735.13...\). \(800 * (330/360) = 733.33\). It seems the problem might have intended to use v=330m/s. However, based on the given values, 735.13 Hz is the calculated answer, and 733.3 Hz is the closest option.


Step 4: Final Answer:

The frequency of sound heard by the listener is approximately 733.3 Hz.
Quick Tip: To remember the signs in the Doppler formula, think logically. If the source and listener are moving closer, the frequency should increase (so make the numerator larger and/or the denominator smaller). If they are moving apart, the frequency should decrease (so make the numerator smaller and/or the denominator larger).

Step 1: Understanding the Question:

We need to analyze the thermodynamics of ice melting at its standard melting point and pressure. Specifically, we need to determine the nature of the work done and the change in internal energy.


Step 2: Key Formula or Approach:

1. Anomalous Expansion of Water: Water is one of the few substances that is denser in its liquid state than in its solid state. This means when ice melts, its volume decreases.

2. Work Done: The work done by a system on its surroundings at constant pressure is given by \(W_{by} = P \Delta V = P(V_{final} - V_{initial})\). The work done on the system is \(W_{on} = -W_{by}\).

3. First Law of Thermodynamics: The change in internal energy is \(\Delta U = Q - W_{by}\), where \(Q\) is the heat added to the system.


Step 3: Detailed Explanation:

When the slab of ice melts, it turns into water. Due to the anomalous property of water, the volume of the resulting water is less than the volume of the initial ice.

- Initial volume = \(V_{ice}\)

- Final volume = \(V_{water}\)

- \(V_{water} < V_{ice}\)

Therefore, the change in volume of the system is negative:
\[ \Delta V = V_{final} - V_{initial} = V_{water} - V_{ice} < 0 \]

Now, let's analyze the work done. The work done by the system on the atmosphere is:
\[ W_{by} = P \Delta V \]

Since \(P\) is positive and \(\Delta V\) is negative, \(W_{by}\) is negative. This means the system does negative work on the atmosphere. Option (A) is incorrect.

The work done on the system by the atmosphere is:
\[ W_{on} = -W_{by} = - (P \Delta V) \]

Since \(P \Delta V\) is negative, \(W_{on}\) is positive. This means positive work is done on the ice-water system by the atmosphere. Option (B) is correct and Option (C) is incorrect.

Let's check the internal energy. Melting is a phase change that requires the absorption of heat (latent heat of fusion), so the heat added to the system, \(Q\), is positive.

From the first law of thermodynamics:
\[ \Delta U = Q - W_{by} \]

Since \(Q > 0\) and \(W_{by} < 0\), we have:
\[ \Delta U = (positive) - (negative) > 0 \]

The internal energy of the ice-water system increases. Therefore, Option (D) is incorrect.


Step 4: Final Answer:

Positive work is done on the ice-water system by the atmosphere.
Quick Tip: The key to this problem is remembering that ice is less dense than water. This means ice melting is a process where the volume contracts. When a system's volume contracts, the surroundings do positive work on the system.

Step 1: Understanding the Question:

We are analyzing a thermodynamic process where a gas is compressed and then heated. We need to find the total change in its internal energy.


Step 2: Key Formula or Approach:

We will use the First Law of Thermodynamics, which states that the change in internal energy (\(\Delta U\)) of a system is equal to the heat added to the system (\(Q\)) minus the work done by the system (\(W\)).
\[ \Delta U = Q - W \]

The work done by the gas during a constant pressure (isobaric) process is given by:
\[ W = P \Delta V = P(V_{final} - V_{initial}) \]


Step 3: Detailed Explanation:

First, let's identify the given quantities:

- Heat added to the gas, \(Q = +100\) J.

- Constant pressure, \(P = 50\) N/m\(^2\).

- Initial volume, \(V_{initial} = 10\) m\(^3\).

- Final volume, \(V_{final} = 4\) m\(^3\).

Next, calculate the work done by the gas:
\[ W = P(V_{final} - V_{initial}) = 50 N/m^2 \times (4 m^3 - 10 m^3) \]
\[ W = 50 \times (-6) = -300 J \]

The negative sign indicates that work is not done by the gas, but rather work is done on the gas during compression.

Now, apply the First Law of Thermodynamics to find the change in internal energy:
\[ \Delta U = Q - W \]
\[ \Delta U = 100 J - (-300 J) \]
\[ \Delta U = 100 + 300 = 400 J \]

Since \(\Delta U\) is positive, the internal energy increases.


Step 4: Final Answer:

The internal energy of the gas increases by 400 J.
Quick Tip: Be very careful with the sign conventions in thermodynamics. \(Q\) is positive when heat is added to the system. \(W\) is positive when work is done by the system. In this case, compression means the system's volume decreases (\(\Delta V < 0\)), so the work done by the system is negative.

Step 1: Understanding the Question:

An ideal gas initially in one container is allowed to expand into an empty (evacuated) container. We need to find the final pressure of the gas.


Step 2: Key Formula or Approach:

This process is a free expansion of an ideal gas. Since no heat is exchanged and no work is done, the temperature of the ideal gas remains constant. Therefore, we can apply Boyle's Law.

Boyle's Law states that for a fixed amount of gas at constant temperature, the pressure and volume are inversely proportional:
\[ P_1 V_1 = P_2 V_2 \]


Step 3: Detailed Explanation:

Let's define the initial and final states of the gas.

Initial State:

- Initial pressure, \(P_1 = 760\) mm of Hg.

- Initial volume, \(V_1 = 10\) liters.

Final State:

- The gas expands to occupy both vessels. So, the final volume is the sum of the volumes of the two vessels.

- Final volume, \(V_2 = 10 liters + 9 liters = 19\) liters.

- Final pressure, \(P_2\), is what we need to find.

Apply Boyle's Law:
\[ P_1 V_1 = P_2 V_2 \]
\[ (760 mm of Hg) \times (10 L) = P_2 \times (19 L) \]

Solve for \(P_2\):
\[ P_2 = \frac{760 \times 10}{19} \]
\[ P_2 = \frac{7600}{19} \]
\[ P_2 = 400 mm of Hg \]


Step 4: Final Answer:

The resultant pressure is 400 mm of Hg.
Quick Tip: In problems where a gas expands into an evacuated container, the final volume is the total volume of all connected containers. Assuming the temperature is constant (which is usually the case for ideal gas free expansion), Boyle's law is the direct way to find the final pressure.

Step 1: Understanding the Question:

We are asked to predict the outcome when a sealed jar completely filled with water is cooled to its freezing point.


Step 2: Key Formula or Approach:

The solution relies on the physical property of water known as anomalous expansion. Unlike most substances, water expands when it freezes into ice.


Step 3: Detailed Explanation:

Water exhibits a unique behavior regarding its density and temperature. Most substances contract upon cooling and solidifying. However, water contracts as it cools from higher temperatures down to 4°C, where it reaches its maximum density. As it cools further from 4°C to 0°C, it begins to expand slightly.

The most significant change occurs during the phase transition from liquid water to solid ice at 0°C. During freezing, the water molecules arrange themselves into a crystalline lattice structure (hexagonal) which is less dense than liquid water. This results in a significant increase in volume, approximately by 9%.

Since the glass jar is sealed and completely full of water, there is no empty space to accommodate this expansion. The expanding ice exerts an immense pressure on the inner walls of the glass jar. This pressure, known as frost wedging or cryostatic pressure, is strong enough to overcome the tensile strength of the glass, causing the jar to crack and break.


Step 4: Final Answer:

The glass jar breaks due to the formation of ice.
Quick Tip: The anomalous expansion of water upon freezing is a fundamental concept with many real-world consequences, such as pipes bursting in winter, the weathering of rocks, and the fact that ice floats on water, which is crucial for aquatic life in cold climates.

Step 1: Understanding the Question:

An air bubble rises from the bottom of a lake to the surface. We need to find the factor by which its volume increases.


Step 2: Key Formula or Approach:

As the bubble rises, the external pressure on it decreases, causing it to expand. Assuming the temperature of the lake water is constant, we can apply Boyle's Law: \(P_1 V_1 = P_2 V_2\). We need to find the pressures at the bottom and at the surface. The pressure is conveniently given in terms of 'meters of water'.


Step 3: Detailed Explanation:

Let the state at the bottom be 1 and at the surface be 2.

Pressure at the surface (\(P_2\)):

This is just the atmospheric pressure.
\[ P_2 = P_{atm} = 10 m of water \]

Pressure at the bottom (\(P_1\)):

This is the sum of the atmospheric pressure and the gauge pressure due to the water column.
\[ P_1 = P_{atm} + P_{gauge} = P_{atm} + h \]
\[ P_1 = 10 m of water + 90 m of water = 100 m of water \]

Let the volume at the bottom be \(V_1\) and at the surface be \(V_2\).

According to Boyle's Law:
\[ P_1 V_1 = P_2 V_2 \]
\[ (100) \times V_1 = (10) \times V_2 \]

We want to find the ratio \(\frac{V_2}{V_1}\), which tells us how many times the volume becomes.
\[ \frac{V_2}{V_1} = \frac{100}{10} = 10 \]

So, \(V_2 = 10 V_1\). The volume becomes 10 times its original volume.


Step 4: Final Answer:

The volume of the bubble becomes 10 times larger.
Quick Tip: When pressure is given in "meters of water," it simplifies calculations. The total pressure at a depth 'h' is simply (Atmospheric pressure in m of water + h). This avoids having to use the formula \(P=\rho g h\) explicitly.

Step 1: Understanding the Question:

The question asks for the underlying physics principle of an endoscope.


Step 2: Key Formula or Approach:

This is a knowledge-based question about the application of optical phenomena. An endoscope uses optical fibers to transmit images from inside the body to an external viewer. We need to identify the principle that allows light to be guided along a curved fiber.


Step 3: Detailed Explanation:

An endoscope consists of a bundle of flexible optical fibers. These fibers are designed to guide light over long distances, even along curved paths. The principle that makes this possible is Total Internal Reflection (TIR).

An optical fiber consists of a core material with a high refractive index (\(n_1\)) surrounded by a cladding material with a slightly lower refractive index (\(n_2\)). Light is introduced into one end of the fiber. As the light travels down the fiber, it strikes the core-cladding boundary at an angle of incidence that is greater than the critical angle.

When the angle of incidence is greater than the critical angle, the light does not refract out of the core into the cladding. Instead, it is completely reflected back into the core. This process repeats itself along the length of the fiber, trapping the light and guiding it to the other end with very minimal loss of intensity. This allows a clear image of the internal organs to be transmitted to the physician's eyepiece or a camera.

- Refraction (A) is the bending of light, but it's TIR (a specific case of refraction and reflection) that is the key.

- Reflection (B) is too general.

- Dispersion (C) is the splitting of light into colors, which is not the primary principle here.


Step 4: Final Answer:

The endoscope is based on the principle of total internal reflection.
Quick Tip: Total Internal Reflection (TIR) is the principle behind several important technologies, including optical fibers (used in endoscopy and telecommunications), sparkling diamonds, and reflecting prisms used in binoculars and periscopes.

Step 1: Understanding the Question:

We are given the wavelength of incident light and the work function of a metal. We need to find the kinetic energy of the photoelectrons emitted.


Step 2: Key Formula or Approach:

We use Einstein's photoelectric equation:
\[ KE_{max} = E_{photon} - \phi \]

where \(KE_{max}\) is the maximum kinetic energy of the emitted electron, \(E_{photon}\) is the energy of the incident photon, and \(\phi\) is the work function of the material.

The energy of a photon can be calculated from its wavelength \(\lambda\). A very useful shortcut formula for this is:
\[ E_{photon} (in eV) = \frac{12400}{\lambda (in Angstroms)} \]


Step 3: Detailed Explanation:

Given values are:

- Wavelength of light, \(\lambda = 5000\) Å.

- Work function, \(\phi = 1.9\) eV.

First, calculate the energy of the incident photons in eV using the shortcut formula:
\[ E_{photon} = \frac{12400}{5000} eV \]
\[ E_{photon} = \frac{12.4}{5} = 2.48 eV \]

Now, use the photoelectric equation to find the maximum kinetic energy:
\[ KE_{max} = E_{photon} - \phi \]
\[ KE_{max} = 2.48 eV - 1.9 eV \]
\[ KE_{max} = 0.58 eV \]


Step 4: Final Answer:

The kinetic energy of the emitted photoelectron will be 0.58 eV.
Quick Tip: The formula \(E(eV) = \frac{12400}{\lambda(\AA)}\) is a lifesaver in exams for photoelectric effect problems. It avoids the need to use \(E = hc/\lambda\) with fundamental constants and the conversion from Joules to electron-volts, saving significant time and reducing calculation errors.

Step 1: Understanding the Question:

We need to examine the ground state electron configurations of the first 20 elements (from Hydrogen to Calcium) and count how many of them have exactly one unpaired electron.


Step 2: Key Formula or Approach:

We will list the elements and their electron configurations, then inspect the outermost orbital to count the unpaired electrons. An unpaired electron is one that occupies an orbital by itself.


Step 3: Detailed Explanation:

Let's list the elements whose configurations result in one unpaired electron:

1. Z=1, Hydrogen (H): \(1s^1\). The single electron in the 1s orbital is unpaired. (1)

2. Z=3, Lithium (Li): \([He] 2s^1\). The single electron in the 2s orbital is unpaired. (2)

3. Z=5, Boron (B): \([He] 2s^2 2p^1\). The single electron in the 2p subshell is unpaired. (3)

4. Z=9, Fluorine (F): \([He] 2s^2 2p^5\). The 2p subshell has three orbitals. The configuration is \(\uparrow\downarrow, \uparrow\downarrow, \uparrow\). There is one unpaired electron. (4)

5. Z=11, Sodium (Na): \([Ne] 3s^1\). The single electron in the 3s orbital is unpaired. (5)

6. Z=13, Aluminum (Al): \([Ne] 3s^2 3p^1\). The single electron in the 3p subshell is unpaired. (6)

7. Z=17, Chlorine (Cl): \([Ne] 3s^2 3p^5\). The 3p subshell has the configuration \(\uparrow\downarrow, \uparrow\downarrow, \uparrow\). There is one unpaired electron. (7)

8. Z=19, Potassium (K): \([Ar] 4s^1\). The single electron in the 4s orbital is unpaired. (8)


Other elements:



Noble gases (He, Ne, Ar) have 0 unpaired electrons.

Alkaline earth metals (Be, Mg, Ca) have 0 unpaired electrons (\(ns^2\)).
Carbon (\(Z = 6\), \(2p^2\)) has 2 unpaired electrons.
Nitrogen (\(Z = 7\), \(2p^3\)) has 3 unpaired electrons.
Oxygen (\(Z = 8\), \(2p^4\)) has 2 unpaired electrons.
Silicon (\(Z = 14\), \(3p^2\)) has 2 unpaired electrons.
Phosphorus (\(Z = 15\), \(3p^3\)) has 3 unpaired electrons.
Sulfur (\(Z = 16\), \(3p^4\)) has 2 unpaired electrons.
Scandium (Z=21) is outside the range.


Counting the elements with exactly one unpaired electron, we have a total of 8.


Step 4: Final Answer:

There are 8 elements with only one unpaired electron in their ground state from Z=1 to Z=20.
Quick Tip: The elements with one unpaired electron are typically in Group 1 (alkali metals), Group 13 (Boron group), and Group 17 (halogens). Quickly identifying these groups can help you count the elements faster.

Step 1: Understanding the Question:

We need to identify which of the given atomic orbitals has its electron density lobes located between the coordinate axes, rather than directly along them.


Step 2: Key Formula or Approach:

This requires knowledge of the standard shapes and orientations of p and d atomic orbitals.

- p orbitals (\(p_x, p_y, p_z\)): These are dumbbell-shaped, and their lobes lie directly along the corresponding axis (x, y, or z).

- d orbitals: There are five d orbitals with two main groups:

- Axial orbitals (\(d_{z^2}, d_{x^2-y^2}\)): Their lobes lie along the axes. \(d_{x^2-y^2}\) has lobes on the x and y axes. \(d_{z^2}\) has a main lobe along the z-axis and a torus in the xy-plane.

- Non-axial orbitals (\(d_{xy}, d_{yz}, d_{xz}\)): These are cloverleaf-shaped, and their lobes lie in the planes indicated by their subscripts, but positioned *between* the axes.


Step 3: Detailed Explanation:

Let's analyze the options:

- (A) \(p_x\): The two lobes of the \(p_x\) orbital lie directly on the x-axis.

- (B) \(p_y\): The two lobes of the \(p_y\) orbital lie directly on the y-axis.

- (C) \(d_{x^2-y^2}\): The four lobes of this orbital lie directly on the x and y axes.

- (D) \(d_{yz}\): The four lobes of this orbital lie in the yz-plane, but they are oriented at 45° to the y and z axes, i.e., between the axes.

Therefore, the \(d_{yz}\) orbital has lobes that are not oriented on the axes.


Step 4: Final Answer:

The orbital \(d_{yz}\) has lobes not orienting on the axis.
Quick Tip: A simple mnemonic for d-orbitals: if the subscript has two different letters (like xy, yz, xz), the lobes are *between* those axes. If the subscript involves squares (like \(x^2-y^2\), \(z^2\)), the lobes are *on* the axes.

Step 1: Understanding the Question:

We are given four sets of quantum numbers (n, l, m, s) and we need to identify which set violates the rules governing these numbers.


Step 2: Key Formula or Approach:

The rules for the quantum numbers are:

1. Principal quantum number (n): Can be any positive integer (1, 2, 3, ...).

2. Azimuthal quantum number (l): Can be any integer from 0 to n-1.

3. Magnetic quantum number (m): Can be any integer from -l to +l, including 0.

4. Spin quantum number (s): Can be +1/2 or -1/2.


Step 3: Detailed Explanation:

Let's check each set against the rules:

- (A) n=2, l=0, m=-1, s=+1/2:

- n=2 is valid.

- l=0 is valid (since \(0 \le 0 \le 2-1\)).

- m=-1 is invalid. For l=0, the only possible value for m is 0. Since the rule is violated, this set is impossible.

- (B) n=3, l=0, m=0, s=-1/2:

- n=3 is valid.

- l=0 is valid (since \(0 \le 0 \le 3-1\)).

- m=0 is valid (since for l=0, m must be 0).

- s=-1/2 is valid. This set is possible (it describes an electron in the 3s orbital).

- (C) n=4, l=1, m=+1, s=+1/2:

- n=4 is valid.

- l=1 is valid (since \(0 \le 1 \le 4-1\)).

- m=+1 is valid (since for l=1, m can be -1, 0, +1).

- s=+1/2 is valid. This set is possible (it describes an electron in a 4p orbital).

- (D) n=3, l=2, m=-1, s=-1/2:

- n=3 is valid.

- l=2 is valid (since \(0 \le 2 \le 3-1\)).

- m=-1 is valid (since for l=2, m can be -2, -1, 0, +1, +2).

- s=-1/2 is valid. This set is possible (it describes an electron in a 3d orbital).


Step 4: Final Answer:

The impossible quantum number set is (2, 0, -1, +1/2).
Quick Tip: The most common errors in quantum number sets involve the 'l' and 'm' values. Always check them sequentially: first, is 'l' valid for the given 'n'? Second, is 'm' valid for the given 'l'? This structured check helps to quickly spot the error.

Step 1: Understanding the Question:

We are given a set of atomic numbers and need to find how many unique binary ionic compounds can be formed between them. An ionic compound is typically formed between a metal (cation) and a non-metal (anion).


Step 2: Key Formula or Approach:

1. Identify the elements from their atomic numbers.

2. Classify each element as a metal or a non-metal. Metals tend to lose electrons (form cations), while non-metals tend to gain electrons (form anions).

3. Count the number of possible combinations between one metal and one non-metal.


Step 3: Detailed Explanation:

1. Identify the elements:

- Z = 8 is Oxygen (O)

- Z = 9 is Fluorine (F)

- Z = 11 is Sodium (Na)

- Z = 19 is Potassium (K)

- Z = 20 is Calcium (Ca)

2. Classify the elements:

- Non-metals (form anions): Oxygen (O\(^{2-}\)), Fluorine (F\(^{-}\)) -- [2 non-metals]

- Metals (form cations): Sodium (Na\(^{+}\)), Potassium (K\(^{+}\)), Calcium (Ca\(^{2+}\)) -- [3 metals]

3. Count the combinations:

We can form an ionic compound by pairing each metal with each non-metal.

- Combinations with Sodium (Na):

- Na and O \(\rightarrow\) Na\(_2\)O (Sodium oxide)

- Na and F \(\rightarrow\) NaF (Sodium fluoride)

- Combinations with Potassium (K):

- K and O \(\rightarrow\) K\(_2\)O (Potassium oxide)

- K and F \(\rightarrow\) KF (Potassium fluoride)

- Combinations with Calcium (Ca):

- Ca and O \(\rightarrow\) CaO (Calcium oxide)

- Ca and F \(\rightarrow\) CaF\(_2\) (Calcium fluoride)

The total number of possible ionic compounds is the number of metals multiplied by the number of non-metals: \(3 \times 2 = 6\).


Step 4: Final Answer:

The number of possible ionic compounds is 6.
Quick Tip: To solve this quickly, just classify the elements into metals and non-metals. The total number of binary ionic compounds will be (number of metals) \(\times\) (number of non-metals).

Step 1: Understanding the Question:

We need to find the molecule from the given options where the ratio of the total number of lone pairs of electrons to the total number of bonding pairs of electrons is 2:3.


Step 2: Key Formula or Approach:

For each molecule, we will draw the Lewis structure to determine the number of lone pairs and bonding pairs. A bonding pair is a pair of electrons shared between two atoms (a single bond is 1 BP, a double bond is 2 BP, a triple bond is 3 BP). A lone pair is a pair of valence electrons that is not shared.


Step 3: Detailed Explanation:

Let's analyze each molecule:

- (A) Cl\(_2\): The Lewis structure is :Cl-Cl:. Each chlorine atom has 3 lone pairs, and there is 1 single bond between them.

- Total Lone Pairs (LP) = 3 + 3 = 6

- Total Bonding Pairs (BP) = 1

- Ratio LP:BP = 6:1.

- (B) O\(_2\): The Lewis structure is :O=O:. Each oxygen atom has 2 lone pairs, and there is 1 double bond between them.

- Total LP = 2 + 2 = 4

- Total BP = 2 (a double bond counts as two pairs)

- Ratio LP:BP = 4:2 = 2:1.

- (C) HCl: The Lewis structure is H-Cl:. The chlorine atom has 3 lone pairs, hydrogen has none. There is 1 single bond.

- Total LP = 3

- Total BP = 1

- Ratio LP:BP = 3:1.

- (D) N\(_2\): The Lewis structure is :N\(\equiv\)N:. Each nitrogen atom has 1 lone pair, and there is 1 triple bond between them.

- Total LP = 1 + 1 = 2

- Total BP = 3 (a triple bond counts as three pairs)

- Ratio LP:BP = 2:3.

This matches the required ratio.


Step 4: Final Answer:

The molecule with a lone pair to bond pair ratio of 2:3 is N\(_2\).
Quick Tip: Drawing the Lewis structure is essential for this type of question. Remember to count bonding pairs based on the bond order: a single bond is 1 pair, a double bond is 2 pairs, and a triple bond is 3 pairs.

Step 1: Understanding the Question:

We are given the molarity and volume of a urea solution and asked to calculate the number of moles of urea.


Step 2: Key Formula or Approach:

Molarity (M) is defined as the number of moles of solute per liter of solution.
\[ Molarity (M) = \frac{moles of solute}{Volume of solution in Liters (L)} \]

We can rearrange this formula to solve for the moles of solute:
\[ moles = Molarity \times Volume (L) \]


Step 3: Detailed Explanation:

Given values are:

- Molarity = 0.2 M (which means 0.2 moles/liter)

- Volume = 250 ml

First, we must convert the volume from milliliters (ml) to liters (L):
\[ Volume (L) = 250 ml \times \frac{1 L}{1000 ml} = 0.250 L \]

Now, use the rearranged formula to calculate the number of moles:
\[ moles = 0.2 \frac{mol}{L} \times 0.250 L \]
\[ moles = 0.05 mol \]


Step 4: Final Answer:

There are 0.05 moles of urea present in the solution.
Quick Tip: A common mistake in molarity calculations is forgetting to convert the volume to liters. Always ensure your units are consistent before multiplying. Molarity is always in moles per liter.

Step 1: Understanding the Question:

This is a dilution problem. A concentrated solution of NaOH is diluted with water to a larger volume and lower concentration. We need to find the initial volume of the concentrated solution.


Step 2: Key Formula or Approach:

When a solution is diluted, the amount (moles) of solute remains constant. This is expressed by the dilution equation:
\[ M_1 V_1 = M_2 V_2 \]

where \(M_1\) and \(V_1\) are the initial molarity and volume, and \(M_2\) and \(V_2\) are the final molarity and volume.


Step 3: Detailed Explanation:

Let's identify the given values:

- Initial Molarity, \(M_1 = 0.1\) M

- Initial Volume, \(V_1 = x\) ml (this is what we need to find)

- Final Molarity, \(M_2 = 0.01\) M

- Final Volume, \(V_2 = 250\) ml

Now, substitute these values into the dilution equation:
\[ M_1 V_1 = M_2 V_2 \]
\[ (0.1 M) \times (x ml) = (0.01 M) \times (250 ml) \]

Now, solve for \(x\):
\[ 0.1 \cdot x = 2.5 \]
\[ x = \frac{2.5}{0.1} = 25 \]

The value of x is 25 ml.


Step 4: Final Answer:

The value of x is 25 ml.
Quick Tip: The dilution formula \(M_1V_1 = M_2V_2\) is fundamental for solving problems involving the dilution of solutions. As long as the units of volume (\(V_1\) and \(V_2\)) are the same (e.g., both in ml or both in L), you don't need to convert them.

Step 1: Understanding the Question:

We need to calculate the normality of a sodium carbonate solution, given the number of molecules, volume, and molecular weight.


Step 2: Key Formula or Approach:

1. Calculate the number of moles of Na\(_2\)CO\(_3\).

2. Calculate the Molarity (M) of the solution.

3. Calculate the Normality (N) using the formula \(N = M \times n-factor\).


Step 3: Detailed Explanation:

1. Calculate moles of Na\(_2\)CO\(_3\):

The number of moles is the number of molecules divided by Avogadro's number (\(N_A\)).
\[ moles = \frac{Number of molecules}{N_A} = \frac{3 \times 10^{22}}{6 \times 10^{23}} = \frac{3}{60} = \frac{1}{20} = 0.05 mol \]

2. Calculate Molarity (M):

Molarity is moles of solute per liter of solution. The volume is 500 ml = 0.5 L.
\[ M = \frac{moles}{Volume (L)} = \frac{0.05 mol}{0.5 L} = 0.1 M \]

3. Calculate Normality (N):

Normality is Molarity times the n-factor. For a salt like Na\(_2\)CO\(_3\), the n-factor is the total positive (or negative) charge of the ions it dissociates into.

Na\(_2\)CO\(_3 \rightarrow 2Na^+ + CO_3^{2-}\)

The total positive charge is \(2 \times (+1) = 2\). The total negative charge is \(-2\). So, the n-factor is 2.
\[ N = M \times n-factor = 0.1 M \times 2 = 0.2 N \]


Step 4: Final Answer:

The normality of the solution is 0.2 N.
Quick Tip: The \(n\)-factor is crucial for converting between molarity and normality. For acids, it is the number of \(H^+\) ions; for bases, the number of \(OH^-\) ions; and for salts, it is the total charge on the cations (or anions).

Step 1: Understanding the Question:

We need to identify the pair of chemical species where both members are Lewis acids.


Step 2: Key Formula or Approach:

- A Lewis acid is a chemical species that can accept a pair of electrons. Common examples include cations, molecules with an incomplete octet, and molecules where the central atom can expand its octet.

- A Lewis base is a chemical species that can donate a pair of electrons. Common examples include anions and molecules with lone pairs of electrons on the central atom.


Step 3: Detailed Explanation:

Let's analyze each species in the options:

- BF\(_3\): Boron trifluoride. The boron atom has only 6 valence electrons (an incomplete octet), so it can accept an electron pair. It is a Lewis acid.

- NH\(_3\): Ammonia. The nitrogen atom has a lone pair of electrons that it can donate. It is a Lewis base.

- H\(^+\): A proton. It has an empty 1s orbital and readily accepts an electron pair. It is a Lewis acid.

- F\(^-\): A fluoride ion. It is an anion with four lone pairs of electrons, making it an electron pair donor. It is a Lewis base.

- H\(_2\)O: Water. The oxygen atom has two lone pairs of electrons that it can donate. It is a Lewis base.

- NH\(_4^+\): Ammonium ion. Nitrogen has a full octet and no lone pairs to donate. It is generally not considered a Lewis base. While it is a Brønsted-Lowry acid (can donate a proton), it is not a typical Lewis acid as it cannot accept an electron pair directly without first losing a proton. However, compared to Lewis bases, it is acidic.

Now let's evaluate the pairs:

- (A) BF\(_3\) (acid), NH\(_3\) (base). Incorrect.

- (B) H\(^+\) (acid), BF\(_3\) (acid). Both are Lewis acids. Correct.

- (C) F\(^-\) (base), H\(_2\)O (base). Both are Lewis bases. Incorrect.

- (D) NH\(_4^+\) (Brønsted-Lowry acid), NH\(_3\) (base). Incorrect.


Step 4: Final Answer:

The pair containing only Lewis acids is H\(^+\), BF\(_3\).
Quick Tip: To quickly identify Lewis acids, look for three main categories: 1. Positive ions (like \(H^+\), \(Mg^{2+}\)). 2. Molecules with an incomplete octet on the central atom (like \(BF_3\), \(AlCl_3\)). 3. Molecules with a central atom that can accommodate more than an octet and is bonded to electronegative atoms (like \(SiF_4\), \(PCl_5\)).

Step 1: Understanding the Question:

We are given the mass of sodium hydroxide (NaOH), a strong base, dissolved in a specific volume of solution.

We need to calculate the pH of this solution.


Step 2: Key Formula or Approach:

1. Calculate the molar mass of NaOH.

2. Calculate the number of moles of NaOH from the given mass.

3. Calculate the molarity of the NaOH solution. Since NaOH is a strong base, this concentration is equal to the hydroxide ion concentration, \([OH^-]\).

4. Calculate the pOH using the formula: \( pOH = -\log_{10}[OH^-] \).

5. Calculate the pH using the relationship: \( pH + pOH = 14 \).


Step 3: Detailed Explanation:

1. Molar Mass of NaOH:

The atomic masses are Na = 23, O = 16, H = 1.

Molar Mass of NaOH = \(23 + 16 + 1 = 40\) g/mol.


2. Moles of NaOH:
\[ moles = \frac{mass}{molar mass} = \frac{4 g}{40 g/mol} = 0.1 mol \]


3. Molarity of NaOH solution:

The volume of the solution is 1.0 L.
\[ Molarity [NaOH] = \frac{moles}{Volume (L)} = \frac{0.1 mol}{1.0 L} = 0.1 M \]

Since NaOH is a strong base, it dissociates completely: NaOH \(\rightarrow\) Na\(^+\) + OH\(^-\).

Therefore, the concentration of hydroxide ions is \([OH^-] = 0.1\) M or \(10^{-1}\) M.


4. Calculate pOH:
\[ pOH = -\log_{10}[OH^-] = -\log_{10}(10^{-1}) = -(-1) = 1 \]


5. Calculate pH:
\[ pH = 14 - pOH = 14 - 1 = 13 \]


Step 4: Final Answer:

The pH of the solution is 13.
Quick Tip: For strong bases like NaOH, the concentration of OH\(^-\) is equal to the molarity of the solution.
A quick way to calculate pH for simple concentrations like 0.1 M, 0.01 M, etc., is to first find the pOH, which will be an integer, and then subtract from 14.

Step 1: Understanding the Question:

This question asks for the value of the total charge carried by one mole of electrons.

This quantity is a fundamental constant in chemistry and physics known as the Faraday constant (F).


Step 2: Key Formula or Approach:

The Faraday constant is calculated by multiplying the charge of a single electron (the elementary charge, \(e\)) by Avogadro's number (\(N_A\)).
\[ F = e \times N_A \]


Step 3: Detailed Explanation:

The standard values for the constants are:

- Charge of one electron, \(e \approx 1.602 \times 10^{-19}\) Coulombs (C).

- Avogadro's number, \(N_A \approx 6.022 \times 10^{23}\) mol\(^{-1}\).

Now, we calculate the Faraday constant:
\[ F = (1.602 \times 10^{-19} C) \times (6.022 \times 10^{23} mol^{-1}) \]
\[ F \approx 9.6485 \times 10^4 C/mol \]

This value is commonly approximated for calculations as 96500 C/mol.

Writing this approximation in scientific notation gives:
\[ 96500 = 9.65 \times 10^4 \]

This matches option (B).


Step 4: Final Answer:

The number of coulombs corresponding to 1 mole of electrons is approximately \(9.65 \times 10^4\).
Quick Tip: The value of the Faraday constant, approximately 96500 C/mol, is one of the most important constants in electrochemistry.
It is essential to memorize this value for quick calculations in electrolysis and electrochemical cell problems.

Step 1: Understanding the Question:

We need to identify which of the given substances, when dissolved in water, does not form an electrically conducting solution.

Such a substance is known as a non-electrolyte.


Step 2: Key Formula or Approach:

An electrolyte is a substance that produces ions when dissolved in a solvent (like water), allowing the solution to conduct electricity.

- Electrolytes are typically ionic compounds (salts), acids, and bases.

- Non-electrolytes are typically molecular covalent compounds that do not ionize in solution, such as sugars, alcohols, and urea.


Step 3: Detailed Explanation:

Let's analyze each option:

- (A) Urea \((CO(NH_2)_2)\): Urea is a covalent organic molecule. When it dissolves in water, it disperses as neutral molecules. It does not dissociate into ions. Therefore, its aqueous solution does not conduct electricity, making it a non-electrolyte.

- (B) Copper Sulphate \((CuSO_4)\): This is an ionic salt. It dissolves in water and dissociates into mobile ions: \(Cu^{2+}(aq)\) and \(SO_4^{2-}(aq)\). It is a strong electrolyte.

- (C) Silver Nitrate (AgNO\(_3\)): This is an ionic salt. It dissolves and dissociates into Ag\(^+\)(aq) and NO\(_3^-\)(aq) ions. It is a strong electrolyte.

- (D) Sodium Chloride (NaCl): This is a common ionic salt that dissolves and dissociates completely into Na\(^+\)(aq) and Cl\(^-\)(aq) ions. It is a strong electrolyte.


Step 4: Final Answer:

The aqueous solution of Urea does not act as an electrolyte.
Quick Tip: To identify non-electrolytes, look for molecular covalent compounds that are not acids or bases.
Common examples include sugars (like glucose, sucrose), alcohols (like ethanol), and urea.
Ionic salts are almost always strong electrolytes.

Step 1: Understanding the Question:

This is an electrolysis problem where we need to calculate the mass of silver metal deposited by a specific amount of electric charge.


Step 2: Key Formula or Approach:

We will use Faraday's Laws of Electrolysis.

1. Write the reduction half-reaction for silver ions to determine the number of electrons involved per ion.

2. Calculate the moles of electrons corresponding to the given charge using the Faraday constant.

3. Use the stoichiometry from the half-reaction to find the moles of silver deposited.

4. Convert the moles of silver to mass, and then convert the units to milligrams.


Step 3: Detailed Explanation:

The reduction half-reaction for silver ions at the cathode is:
\[ Ag^+(aq) + e^- \rightarrow Ag(s) \]

This shows that 1 mole of electrons (e\(^-\)) deposits 1 mole of silver (Ag).


First, calculate the moles of electrons for the given charge Q = 9.65 C.
\[ Moles of electrons = \frac{Total Charge (Q)}{Faraday Constant (F)} = \frac{9.65 C}{96500 C/mol} = 0.0001 mol = 10^{-4} mol \]


From the 1:1 stoichiometry, the moles of Ag deposited equals the moles of electrons.
\[ Moles of Ag = 10^{-4} mol \]


Next, convert the moles of silver to mass in grams using the molar mass (108 g/mol).
\[ Mass of Ag (g) = moles \times molar mass = 10^{-4} mol \times 108 g/mol = 0.0108 g \]


Finally, convert the mass from grams (g) to milligrams (mg) by multiplying by 1000.
\[ Mass of Ag (mg) = 0.0108 g \times 1000 mg/g = 10.8 mg \]


Step 4: Final Answer:

The amount of silver deposited is 10.8 mg.
Quick Tip: A useful proportion to remember for electrolysis is:
\( \frac{Mass deposited}{Molar mass} = \frac{Charge passed}{n \times F} \)
where 'n' is the number of moles of electrons per mole of substance from the half-reaction.

Step 1: Understanding the Question:

We are given standard reduction potentials (E°) for three metals. We must determine which statement about their spontaneous redox behavior is correct.


Step 2: Key Formula or Approach:

The standard reduction potential (E°) measures the tendency for a species to be reduced.

- A more negative E° indicates a stronger reducing agent (the metal is more easily oxidized).

- A more positive E° indicates a stronger oxidizing agent (the ion is more easily reduced).

A metal can spontaneously reduce the ions of another metal that has a more positive E°.


Step 3: Detailed Explanation:

Let's list the potentials in increasing order to create an electrochemical series:

E°(Zn\(^{2+}\)/Zn) = -0.76 V

E°(Cu\(^{2+}\)/Cu) = +0.34 V

E°(Ag\(^+\)/Ag) = +0.80 V


This order establishes the relative strengths:

- Strength as reducing agents (metals): Zn \(>\) Cu \(>\) Ag.

- Strength as oxidizing agents (ions): Ag\(^+\) \(>\) Cu\(^{2+}\) \(>\) Zn\(^{2+}\).


Now, let's evaluate each statement:

- (A) Ag can oxidize Zn and Cu: Incorrect. The metal Ag is a reducing agent. Its ion, Ag\(^+\), is an oxidizing agent.

- (B) Ag can reduce Zn\(^{2+}\) and Cu\(^{2+}\): Incorrect. Ag is the weakest reducing agent and cannot reduce ions of metals with lower (more negative) E° values.

- (C) Zn can reduce Ag\(^+\) and Cu\(^{2+}\): Correct. Zn is the strongest reducing agent. Its E° is the most negative, so it can spontaneously reduce the ions of both Cu and Ag, which have more positive E° values.

- (D) Cu can oxidize Zn and Ag: Incorrect. The metal Cu is a reducing agent. Its ion, Cu\(^{2+}\), can oxidize Zn but not Ag.


Step 4: Final Answer:

The correct statement is "Zn can reduce Ag\(^+\) and Cu\(^{2+}\)".
Quick Tip: A simple rule to remember is: a metal can "displace" or reduce the ions of any metal below it in the reactivity series (or above it in the standard potential series).
Here, Zn is the most reactive, so it can reduce both Cu\(^{2+}\) and Ag\(^+\).

Step 1: Understanding the Question:

The question asks about the ion-exchange mechanism for softening hard water using the permutit process, specifically identifying the ions removed from the water.


Step 2: Key Formula or Approach:

This question requires factual knowledge of water chemistry.

1. Define the ions responsible for permanent hardness in water.

2. Understand the principle of the permutit (zeolite) ion-exchange process.


Step 3: Detailed Explanation:

1. Hardness of Water: The hardness of water is defined by the presence of dissolved divalent cations, primarily calcium (Ca\(^{2+}\)) and magnesium (Mg\(^{2+}\)). Permanent hardness is caused by their chloride and sulfate salts.


2. Permutit Process: The permutit process uses a synthetic zeolite, which is a hydrated sodium aluminum silicate (often represented as Na\(_2\)Z), as an ion-exchange resin. When hard water passes through a column of this material, the Ca\(^{2+}\) and Mg\(^{2+}\) ions are exchanged for the Na\(^+\) ions in the permutit. The zeolite has a higher affinity for the divalent Ca\(^{2+}\) and Mg\(^{2+}\) ions than for the monovalent Na\(^+\) ions.

The exchange reactions are:
\[ Ca^{2+}(aq) + Na_2Z(s) \rightarrow CaZ(s) + 2Na^+(aq) \]
\[ Mg^{2+}(aq) + Na_2Z(s) \rightarrow MgZ(s) + 2Na^+(aq) \]

The hardness-causing ions are thus trapped by the resin, and sodium ions are released into the water, resulting in soft water.


Step 4: Final Answer:

In the permutit process, Na\(^+\) ions are exchanged with Ca\(^{2+}\) and Mg\(^{2+}\) ions.
Quick Tip: Remember the simple fact: Water hardness is caused by Ca\(^{2+}\) and Mg\(^{2+}\) ions.
Therefore, any water softening process must be designed to remove these two specific ions.

Step 1: Understanding the Question:

We need to calculate the hardness of a water sample in ppm, expressed in terms of CaCO\(_3\) equivalents, given the mass of MgCl\(_2\) in a known mass of water.


Step 2: Key Formula or Approach:

1. Convert the mass of the hardness-causing salt (MgCl\(_2\)) to its equivalent mass of CaCO\(_3\) using the ratio of their molar masses. The molar mass of CaCO\(_3\) is 100 g/mol.

\[ Mass of CaCO_3 equiv. = (Mass of salt) \times \frac{Molar mass of CaCO_3}{Molar mass of salt} \]

2. Calculate the concentration in ppm (parts per million), defined as mg of CaCO\(_3\) equivalent per kg of water.


Step 3: Detailed Explanation:

1. Calculate CaCO\(_3\) equivalent mass:

- Mass of MgCl\(_2\) = 19 mg.

- Molar mass of MgCl\(_2\) = 95 g/mol.

- Molar mass of CaCO\(_3\) = 100 g/mol.

Using the equivalence formula:
\[ Mass of CaCO_3 equiv. = 19 mg \times \frac{100}{95} = \frac{1900}{95} mg = 20 mg \]

This means 19 mg of MgCl\(_2\) creates the same hardness as 20 mg of CaCO\(_3\).


2. Calculate hardness in ppm:

The mass of the water sample is 2 kg.
\[ Hardness (ppm) = \frac{Mass of CaCO_3 equiv. (mg)}{Mass of water (kg)} \]
\[ Hardness (ppm) = \frac{20 mg}{2 kg} = 10 mg/kg \]

Since 1 mg/kg is equivalent to 1 ppm, the hardness is 10 ppm.


Step 4: Final Answer:

The degree of hardness is 10 ppm.
Quick Tip: Hardness calculations always involve converting the given salt into its CaCO\(_3\) equivalent.
The conversion factor is always \(\frac{100}{Molar mass of the salt}\).
Then, remember that ppm = mg/kg (or mg/L for dilute solutions).

Step 1: Understanding the Question:

We need to identify the pair of dissolved chlorides that are responsible for causing permanent hardness in water.


Step 2: Key Formula or Approach:

This question is based on the chemical definition of water hardness.

- Water Hardness: Caused by dissolved divalent cations, primarily Calcium (Ca\(^{2+}\)) and Magnesium (Mg\(^{2+}\)).

- Permanent Hardness: Caused by the chlorides and sulfates of these ions; this type is not removed by boiling.

- Temporary Hardness: Caused by the bicarbonates of these ions.


Step 3: Detailed Explanation:

Let's analyze the pairs in the options:

- (A) NaCl, KCl: Sodium and potassium ions are monovalent and do not cause hardness.

- (B) CaCl\(_2\), KCl: Calcium chloride (CaCl\(_2\)) causes permanent hardness, but potassium chloride (KCl) does not.

- (C) AlCl\(_3\), MgCl\(_2\): Magnesium chloride (MgCl\(_2\)) causes permanent hardness, but aluminum chloride is not considered a cause of common water hardness.

- (D) MgCl\(_2\), CaCl\(_2\): Both magnesium chloride and calcium chloride are the definitive salts responsible for permanent hardness.


Step 4: Final Answer:

The pair of chlorides responsible for permanent hardness of water is MgCl\(_2\), CaCl\(_2\).
Quick Tip: A simple rule to remember for water hardness:
Hardness Ions: Ca\(^{2+}\) and Mg\(^{2+}\).
Anions for Temporary Hardness: Bicarbonate (HCO\(_3^-\)).
Anions for Permanent Hardness: Chloride (Cl\(^-\)) and Sulfate (SO\(_4^{2-}\)).

Step 1: Understanding the Question:

The question asks to identify the type of electrochemical corrosion cell formed in a bent metal pipe.


Step 2: Key Formula or Approach:

This question is about electrochemical corrosion. Corrosion often happens when different areas on a metal surface develop a potential difference, creating a tiny galvanic cell. The cell is named based on the cause of this potential difference.


Step 3: Detailed Explanation:

When a metal pipe is bent, it is subjected to non-uniform mechanical stress.

- The outer curve is under tensile stress (stretched).

- The inner curve is under compressive stress.

A region of a metal under higher stress is more energetically active and thus has a more negative electrode potential. This makes the highly stressed region (the bend) anodic, while the less stressed regions become cathodic.

This potential difference, arising from differences in mechanical stress across the metal surface, creates a galvanic cell known as a Stress Cell. This leads to accelerated corrosion at the stressed areas.


Step 4: Final Answer:

The cell formed in bent pipes is an example of a Stress Cell.
Quick Tip: Remember that non-uniformity is a major cause of corrosion.
- Difference in metal type \(\rightarrow\) Galvanic/Composition Cell.
- Difference in oxygen concentration \(\rightarrow\) Differential Aeration Cell.
- Difference in mechanical stress \(\rightarrow\) Stress Cell.

Step 1: Understanding the Question:

We are asked to identify the chemical compound that forms on silver, causing it to tarnish (turn black).


Step 2: Key Formula or Approach:

This is a factual question about the corrosion of silver. Tarnishing is the chemical reaction between silver and sulfur compounds present in the atmosphere.


Step 3: Detailed Explanation:

Silver (Ag) is a relatively unreactive metal but is susceptible to reaction with sulfur compounds. The atmosphere contains trace amounts of hydrogen sulfide (H\(_2\)S), which originates from pollution and biological decay.

Silver reacts with H\(_2\)S in the presence of oxygen to form a black layer of silver sulfide (Ag\(_2\)S). This layer is the tarnish.

The reaction is:
\[ 4Ag(s) + 2H_2S(g) + O_2(g) \rightarrow 2Ag_2S(s) + 2H_2O(l) \]

This black silver sulfide layer obscures the metal's luster.


Step 4: Final Answer:

Tarnishing of silver is due to the formation of its sulphide layer.
Quick Tip: Remember the colors of common corrosion products:
- Iron \(\rightarrow\) Reddish-brown rust (hydrated iron(III) oxide).
- Copper \(\rightarrow\) Greenish-blue patina (copper carbonate/sulfate).
- Silver \(\rightarrow\) Black tarnish (silver sulfide).

Step 1: Understanding the Question:

We must distinguish between a co-polymer and a homopolymer among the given options.


Step 2: Key Formula or Approach:

- A Homopolymer is a polymer formed from the polymerization of a single type of monomer.

- A Co-polymer is a polymer formed from two or more different types of monomers.

We need to know the monomer(s) for each polymer listed.


Step 3: Detailed Explanation:

Let's analyze the monomers of each polymer:

- (A) Buna-S rubber: This is a co-polymer. Its name is an abbreviation for its monomers: Bu (for 1,3-Butadiene) and Na-S (for Styrene, with Na indicating the original sodium catalyst).

- (B) Neoprene rubber: This is a homopolymer. It is formed by the addition polymerization of a single monomer: chloroprene (2-chloro-1,3-butadiene). Its systematic name is polychloroprene.

- (C) Bakelite: This is a co-polymer. It is a condensation polymer made from two different monomers: Phenol and Formaldehyde.

- (D) Urea - Formaldehyde: As its name clearly indicates, this is a co-polymer made from Urea and Formaldehyde monomers via condensation polymerization.

Therefore, Neoprene rubber is the only homopolymer in the list.


Step 4: Final Answer:

Neoprene rubber is not a co-polymer.
Quick Tip: The names of some polymers give clues to their composition.
"Buna-S" stands for Butadiene-Styrene.
Names like "Urea-Formaldehyde" explicitly state the two monomers.
When a polymer is named after a single monomer, like Polystyrene or Neoprene (polychloroprene), it's a homopolymer.

Step 1: Understanding the Question:

We need to identify the chemical structure of the monomer unit that polymerizes to form polystyrene.


Step 2: Key Formula or Approach:

The name "polystyrene" indicates that it is a polymer ("poly-") made from the monomer "styrene".

We need to know the structure of styrene. Styrene is also known as vinylbenzene or phenylethene.

It consists of a vinyl group (\(-CH=CH_2\)) attached to a phenyl group (a benzene ring, \(C_6H_5\)).


Step 3: Detailed Explanation:

Let's analyze the chemical structures given in the options:

- (A) \(CH_2=CH-Cl\): This is vinyl chloride, the monomer for polyvinyl chloride (PVC).

- (B) \(CH_2=CH-CN\): This is acrylonitrile, the monomer for polyacrylonitrile (PAN), used in fibers like Orlon.

- (C) \(CH_2=CH-C_6H_5\): This is styrene (vinylbenzene). The addition polymerization of this monomer across the double bond results in polystyrene.

- (D) \(CH_2=CH-CH_3\): This is propene (or propylene), the monomer for polypropylene.

Therefore, the correct monomer for polystyrene is styrene, \(CH_2=CH-C_6H_5\).


Step 4: Final Answer:

The monomer involved in the formation of polystyrene is \(CH_2=CH-C_6H_5\).
Quick Tip: Many common polymer names are simply the prefix "poly-" added to the name of the monomer.
For example: poly(ethene), poly(propene), poly(styrene), poly(vinyl chloride).
Recognizing this naming convention can help you quickly identify the monomer.

Step 1: Understanding the Question:

The question asks about the process used to improve the physical properties of natural rubber, such as its strength and elasticity.


Step 2: Key Formula or Approach:

This is a factual question about a specific industrial chemical process. The process of heating natural rubber with a cross-linking agent to improve its properties is known as vulcanization.


Step 3: Detailed Explanation:

Natural rubber is a polymer of isoprene. In its raw form, it has several undesirable properties: it is soft and sticky, has low tensile strength, becomes brittle at low temperatures, and is not very elastic.

To overcome these issues, natural rubber is heated with Sulphur. This process, discovered by Charles Goodyear, is called vulcanization.

During vulcanization, the sulphur atoms form cross-links (disulfide or polysulfide bridges) between the long polyisoprene chains. These cross-links tie the polymer chains together, preventing them from slipping past one another.

This modification drastically improves the rubber's properties, making it stronger, tougher, more elastic, and less susceptible to temperature changes.

Carbon (in the form of carbon black) is often added as a reinforcing filler, but Sulphur is the key vulcanizing agent.


Step 4: Final Answer:

The undesirable properties of natural rubber are overcome by heating it with Sulphur.
Quick Tip: The process of heating rubber with sulfur is called Vulcanization.
Remembering this term and its association with sulfur is key to answering questions about improving rubber's properties.

Step 1: Understanding the Question:

We need to identify the primary chemical components of Liquefied Petroleum Gas (LPG).


Step 2: Key Formula or Approach:

This is a factual question about common fuels. LPG is a byproduct of petroleum refining and natural gas processing. Its composition varies but is primarily composed of low-molecular-weight alkanes.


Step 3: Detailed Explanation:

Liquefied Petroleum Gas (LPG) is a flammable mixture of hydrocarbon gases used as fuel.

The main components of LPG are propane (\(C_3H_8\)) and butane (\(C_4H_{10}\)). Butane itself exists as two isomers: n-butane and isobutane. Commercial LPG is often a mix of these gases.

Let's analyze the options:

- (A) Methane, Ethane: Methane (\(CH_4\)) is the primary component of natural gas, not LPG.

- (B) Ethane, Propane: Propane is a major component, but ethane is typically a minor component. This is less accurate than other options.

- (C) Butane, Isobutane: Both n-butane and its isomer, isobutane, are major components of LPG, often blended with propane. This is the best description among the choices.

- (D) Ethene, Ethyne: These are unsaturated hydrocarbons (alkenes and alkynes) and are not the main constituents of LPG.


Step 4: Final Answer:

LPG mainly contains Butane and Isobutane (along with Propane).
Quick Tip: Remember the main components of common fuel gases:
- \textbf{Natural Gas:} Primarily Methane (C1).
- \textbf{LPG:} Primarily Propane (C3) and Butane (C4).
- \textbf{Gasoline:} A mix of hydrocarbons from C4 to C12.

Step 1: Understanding the Question:

We need to identify which of the given gases is a primary cause of the greenhouse effect.


Step 2: Key Formula or Approach:

The greenhouse effect is the process by which certain gases in the atmosphere trap heat by absorbing infrared radiation emitted from the Earth's surface.

These gases are known as greenhouse gases. To be a greenhouse gas, a molecule must have a changing dipole moment when it vibrates.


Step 3: Detailed Explanation:

Let's analyze the gases:

- The most important greenhouse gases are Water Vapor (\(H_2O\)), Carbon Dioxide (\(CO_2\)), Methane (\(CH_4\)), Nitrous Oxide (\(N_2O\)), and Ozone (\(O_3\)).

- (D) \(CO_2\): Carbon dioxide is a major greenhouse gas. It is the most significant long-lived greenhouse gas contributing to modern climate change.

- (A) \(NO_2\): Nitrogen dioxide is a greenhouse gas but its contribution is less significant than \(CO_2\).

- (B) CO, (C) NO: Carbon monoxide and nitric oxide are not significant greenhouse gases themselves, although they can indirectly affect atmospheric chemistry.

Comparing the options, \(CO_2\) is the most prominent and direct cause of the enhanced greenhouse effect.


Step 4: Final Answer:

The greenhouse effect is caused by \(CO_2\).
Quick Tip: While many gases are present in the atmosphere, the key greenhouse gases to remember are \(H_2O\), \(CO_2\), \(CH_4\), and \(N_2O\).
Symmetrical diatomic molecules like \(N_2\) and \(O_2\) are not greenhouse gases.

Step 1: Understanding the Question:

We need to identify the compound from the list that is the primary cause of the destruction of the stratospheric ozone layer.


Step 2: Key Formula or Approach:

Ozone layer depletion is primarily caused by chemical compounds containing chlorine and bromine that are stable enough to reach the stratosphere, known as chlorofluorocarbons (CFCs).


Step 3: Detailed Explanation:

The stratospheric ozone layer is depleted by catalytic reactions involving free radicals, primarily chlorine (\(Cl\cdot\)) and bromine (\(Br\cdot\)) radicals.

The main source of these halogen radicals is a class of man-made compounds called Chlorofluorocarbons (CFCs).

Let's look at the options:

- (A) \(CO_2\): A greenhouse gas, but does not deplete ozone.

- (B) \(CH_4\): A greenhouse gas, but does not directly deplete ozone.

- (C) \(CH_3OH\) (Methanol): Decomposes in the lower atmosphere and does not significantly affect the ozone layer.

- (D) \(CF_2Cl_2\) (Dichlorodifluoromethane): This is a classic example of a CFC (Freon-12). In the stratosphere, UV radiation breaks its C-Cl bonds, releasing chlorine free radicals that catalytically destroy ozone.


Step 4: Final Answer:

The compound mainly responsible for the depletion of the ozone layer is \(CF_2Cl_2\).
Quick Tip: Remember the distinction:
- \textbf{Greenhouse Effect} (Global Warming) \(\rightarrow\) CO\(_2\), CH\(_4\).
- \textbf{Ozone Depletion} \(\rightarrow\) CFCs (compounds with Chlorine and Fluorine).

Step 1: Understanding the Question:

The question asks to identify the material used for manufacturing ball bearings.


Step 2: Detailed Explanation:

Ball bearings are components that require very specific material properties because they operate under high stress and repetitive motion.

The key properties needed are:


High Hardness: To resist deformation under load.

High Wear Resistance: To prevent material loss due to friction.

High Toughness: To resist fracture from impact loads.

Good Corrosion Resistance: To prevent degradation from environmental factors.



Chrome carbon steel, also known as high-carbon chromium bearing steel, is the most suitable material among the options.


Plain carbon steel lacks the necessary hardness and wear resistance for long-term bearing applications.

Chrome carbon steel contains chromium, which forms hard carbides (like chromium carbide).
These carbides significantly increase the hardness, wear resistance, and toughness of the steel, making it ideal for ball bearings.

Stainless steel has excellent corrosion resistance but generally does not achieve the same level of hardness as specialized bearing steels.
It is used in applications where corrosion is the primary concern.

Malleable cast iron is not hard enough and lacks the rolling fatigue strength required for ball bearings.



Step 3: Final Answer:

Therefore, chrome carbon steel is the standard material for making ball bearings due to its excellent combination of hardness, wear resistance, and toughness.
Quick Tip: For material science questions, focus on linking the application (e.g., ball bearings) to the required material properties (e.g., hardness, wear resistance).
Knowing the effect of alloying elements like chromium is key.

Step 1: Understanding the Question:

The question asks to identify which of the given metals is most susceptible to corrosion when exposed to the atmosphere.


Step 2: Detailed Explanation:

Atmospheric corrosion is an electrochemical process where a metal reacts with its environment (primarily oxygen and moisture) to form a more stable compound, like an oxide or hydroxide.

The tendency of a metal to corrode is related to its position in the electrochemical or reactivity series.
Metals that are more reactive (have a lower reduction potential) corrode more easily.

Let's compare the reactivity of the given metals:


Iron (Fe): Iron is a relatively reactive metal.
In the presence of oxygen and water, it readily oxidizes to form iron(III) oxide, commonly known as rust.
This process is pervasive and destructive.

Copper (Cu): Copper is less reactive than iron.
It corrodes slowly to form a greenish layer called patina (a mix of copper carbonate, sulfate, etc.), which actually protects the underlying metal from further corrosion.

Tin (Sn): Tin is also less reactive than iron.
It is often used as a protective coating for steel (as in tin cans) because it corrodes much more slowly.

Silver (Ag): Silver is a noble metal and is quite unreactive.
It does not rust but can tarnish over time by reacting with sulfur compounds in the air to form black silver sulfide.
However, this is a much slower and less destructive process than the rusting of iron.



Based on the reactivity series, iron is the most reactive among the four options and is therefore the most prone to significant atmospheric corrosion (rusting).


Step 3: Final Answer:

Iron is the most susceptible to atmospheric corrosion among the given choices.
Quick Tip: Remembering the basic reactivity series of common metals (e.g., \(K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au\)) can help you quickly answer questions about corrosion and displacement reactions.

Step 1: Understanding the Question:

The question asks for a material that can safely contain fuming sulfuric acid (oleum), which is a very strong and corrosive oxidizing acid.


Step 2: Detailed Explanation:

Fuming sulfuric acid (\(H_2S_2O_7\)) is a solution of sulfur trioxide (\(SO_3\)) in sulfuric acid (\(H_2SO_4\)).
It is extremely corrosive.

Let's evaluate the options:


Carbon Steel: It is rapidly attacked by concentrated sulfuric acid, especially at elevated temperatures.
It is completely unsuitable for fuming sulfuric acid.

Stainless Steel Type 304: While austenitic stainless steels like 304 have good corrosion resistance in many environments, they are not resistant to hot concentrated sulfuric acid or fuming sulfuric acid.
The acid will break down the passive protective layer.

Nickel: Pure nickel has good resistance to some acids but can be attacked by oxidizing acids.

Monel: Monel is a nickel-copper alloy (typically around 67% Ni and 30% Cu).
It is well-known for its excellent resistance to a wide range of corrosive environments, including hydrofluoric acid and sulfuric acid.
It performs significantly better than pure nickel and stainless steels in these conditions, making it a suitable choice for handling fuming sulfuric acid under certain conditions.


Specialty alloys are required for such harsh chemical services.
Monel's high nickel content provides resistance to reducing conditions, and its overall composition allows it to withstand strong acids better than the other options.


Step 3: Final Answer:

Monel is the most suitable material of construction for use with fuming sulfuric acid among the given options due to its superior corrosion resistance.
Quick Tip: For questions about materials in chemical service, remember that general-purpose materials like carbon steel and SS 304 have limits.
For highly corrosive substances like strong acids, specialty alloys like Monel, Hastelloy, or pure metals like Tantalum are often required.

Step 1: Understanding the Question:

The question asks to identify the primary chemical substance that acts as a reducing agent in the process of extracting iron in a blast furnace.


Step 2: Key Formula or Approach:

In a blast furnace, iron ore (mainly iron(III) oxide, \(Fe_2O_3\)) is converted into molten iron.
This conversion is a reduction reaction, where oxygen is removed from the iron oxide.
A reducing agent is a substance that donates electrons or causes the reduction.


Step 3: Detailed Explanation:

The process inside a blast furnace involves several chemical reactions:


Combustion of Coke: Hot air is blown into the furnace, which reacts with coke (a form of carbon) to produce carbon dioxide and a large amount of heat.

\[ C(s) + O_2(g) \rightarrow CO_2(g) \quad (Exothermic) \]
Formation of Carbon Monoxide: The carbon dioxide produced then rises and reacts with more hot coke to form carbon monoxide.

\[ CO_2(g) + C(s) \rightarrow 2CO(g) \quad (Endothermic) \]
Reduction of Iron Ore: The carbon monoxide is the main reducing agent.
It moves up the furnace and reduces the iron ore (\(Fe_2O_3\)) to molten iron in a series of reactions at different temperatures.
The overall reaction is:

\[ Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(l) + 3CO_2(g) \]

While carbon itself can act as a reducing agent at very high temperatures, the primary reduction throughout the bulk of the furnace is carried out by carbon monoxide gas.


Step 4: Final Answer:

The main reducing agent in an iron blast furnace is carbon monoxide (CO).
Quick Tip: For metallurgical processes, remember the roles of key inputs.
In a blast furnace: Coke is the fuel and source of the reducing agent, Iron Ore is the source of iron, and Limestone is a flux to remove impurities.
The actual reducing agent is CO.

Step 1: Understanding the Question:

The question asks to identify the chemical substance from the options that reacts with glass.


Step 2: Key Formula or Approach:

Glass is primarily composed of silicon dioxide (\(SiO_2\)), which is chemically quite inert and resistant to most acids.
However, it has a specific reaction with one particular acid.

The reaction is:
\[ SiO_2(s) + 4HF(aq) \rightarrow SiF_4(g) + 2H_2O(l) \]

Step 3: Detailed Explanation:


Glass is an amorphous solid made mostly of silica (silicon dioxide, \(SiO_2\)).

Most acids, including strong acids like sulfuric acid (\(H_2SO_4\)) and nitric acid (\(HNO_3\)), do not react with glass.
This is why glass is commonly used for laboratory glassware and chemical storage.

Hydrofluoric acid (HF) is unique in its ability to react with silicon dioxide.
The fluorine in HF attacks the silicon atom, and the hydrogen attacks the oxygen atom, breaking down the \(SiO_2\) structure to form silicon tetrafluoride (a gas) and water.

This reaction is the basis for the industrial process of etching glass to create decorative patterns or frosted surfaces.
Because it attacks glass, HF cannot be stored in glass containers and must be kept in plastic (e.g., polyethylene) bottles.


The other options, such as sulfurous acid (\(H_2SO_3\)), nitric acid (\(HNO_3\)), and potassium dichromate (\(K_2Cr_2O_7\)), do not react with glass.


Step 4: Final Answer:

Glass reacts with hydrofluoric acid (HF).
Quick Tip: Remember the unique property of hydrofluoric acid (HF) to dissolve glass.
This is a very common and important fact in chemistry, often appearing in exams.
This is why HF is stored in plastic containers.

Step 1: Understanding the Question:

The question asks for the temperature at which the eutectic reaction occurs in the iron-carbon (Fe-C) phase diagram.


Step 2: Detailed Explanation:

A eutectic reaction is an invariant reaction in a phase diagram where, upon cooling, a single liquid phase transforms into two solid phases simultaneously.
The reaction can be represented as:
\[ Liquid \xrightarrow{Cooling} Solid_1 + Solid_2 \]
In the iron-carbon system, which is fundamental to understanding steels and cast irons, there are several key invariant reactions:


Eutectic Reaction: This occurs at a specific temperature and composition.
For the Fe-C system, the liquid phase transforms into two solid phases: austenite (\(\gamma\)-iron) and cementite (\(Fe_3C\)).
This reaction happens at \(1147^\circ\)C with a carbon concentration of 4.3 wt%.

\[ Liquid (4.3 wt% C) \rightarrow \gamma-Austenite (2.11 wt% C) + Fe_3C (6.67 wt% C) \]
Eutectoid Reaction: This is a solid-state reaction where one solid phase transforms into two different solid phases.
It occurs at \(727^\circ\)C, where austenite transforms into ferrite and cementite (forming pearlite).

Peritectic Reaction: This reaction involves a liquid and a solid phase transforming into a different solid phase upon cooling.
It occurs at \(1493^\circ\)C in the Fe-C system.


The question specifically asks for the eutectic reaction temperature, which is \(1147^\circ\)C.


Step 3: Final Answer:

The eutectic reaction for the iron-carbon system occurs at \(1147^\circ\)C.
Quick Tip: Memorize the key temperatures and compositions of the iron-carbon diagram: Eutectoid (0.76 wt% C, \(727^\circ\)C) and Eutectic (4.3 wt% C, \(1147^\circ\)C).
Distinguishing between eutectoid (Solid \(\rightarrow\) Solid+Solid) and eutectic (Liquid \(\rightarrow\) Solid+Solid) is crucial.

Step 1: Understanding the Question:

The question asks for the primary purpose of using a bypass stream in a chemical process.


Step 2: Detailed Explanation:

In chemical engineering, process streams are often manipulated to achieve desired outcomes.
Let's analyze the different types of streams:


Bypass Stream: A bypass stream is created by splitting a process stream and routing a portion of it around a piece of equipment (like a reactor or a heat exchanger).
This bypassed portion is then recombined with the stream that has passed through the unit.
The main purpose is to control the composition or properties (like temperature) of the final outlet stream.
By adjusting the fraction of the feed that is bypassed, operators can finely tune the output, effectively controlling the flow streams and their properties.
For example, if a reactor achieves too high a conversion, some of the feed can be bypassed to mix with the highly converted product, thereby lowering the overall conversion to the desired setpoint.

Recycle Stream: A stream where a portion of the product is sent back to be mixed with the fresh feed.
This is primarily used to improve conversion of reactants or to recover valuable catalysts.

Purge Stream: A stream bled off from a recycle loop to prevent the buildup of inert or unwanted substances.
This is used to limit the inerts.


Improving conversion and increasing yield are typically achieved with recycle streams, not bypass streams.
Limiting inerts is the function of a purge stream.
Therefore, the most accurate description of a bypass stream's function is to control the flow streams to regulate the final product's properties.


Step 3: Final Answer:

A bypass stream is used to control the properties of the final output stream, which is a form of controlling the flow streams.
Quick Tip: For chemical process questions, clearly distinguish between the functions of Recycle, Purge, and Bypass streams.
\textbf{Recycle:} Increases conversion.
\textbf{Purge:} Removes inerts.
\textbf{Bypass:} Controls output composition/temperature.

Step 1: Understanding the Question:

The question asks for the mass (weight) in grams of 56 litres of chlorine gas at Standard Temperature and Pressure (S.T.P.).


Step 2: Key Formula or Approach:

We will use the molar volume of a gas at S.T.P. and the molar mass of chlorine gas.


At S.T.P. (Standard Temperature and Pressure: \(0^\circ\)C or 273.15 K and 1 atm), 1 mole of any ideal gas occupies a volume of 22.4 litres.

The number of moles (\(n\)) can be calculated as: \(n = \frac{Given Volume}{Molar Volume at S.T.P.}\).

The mass (\(m\)) can be calculated as: \(m = n \times Molar Mass\).



Step 3: Detailed Explanation:

Part 1: Calculate the molar mass of chlorine gas.

Chlorine is a diatomic molecule, so its chemical formula is \(Cl_2\).

The atomic mass of Chlorine (Cl) is approximately 35.5 g/mol.

Molar mass of \(Cl_2\) = \(2 \times 35.5 = 71.0\) g/mol.

Part 2: Calculate the number of moles of chlorine gas.

Given Volume = 56 litres.

Molar Volume at S.T.P. = 22.4 L/mol.

Number of moles (\(n\)) = \(\frac{56 L}{22.4 L/mol} = 2.5 mol\).

Part 3: Calculate the weight (mass) of the gas.

Mass (\(m\)) = Number of moles (\(n\)) \(\times\) Molar Mass (M).

Mass (\(m\)) = \(2.5 mol \times 71.0 g/mol\).
\[ m = 177.5 g \]

Step 4: Final Answer:

The weight of 56 litres of chlorine gas at S.T.P. is 177.5 g.
Quick Tip: Always remember the standard molar volume of a gas at S.T.P. is 22.4 L/mol.
Also, be careful to use the molecular formula for gases that exist as diatomic molecules (like \(H_2\), \(N_2\), \(O_2\), \(F_2\), \(Cl_2\)).

Step 1: Understanding the Question:

The question asks to identify the part of the van der Waals equation that corrects for the attractive forces between gas molecules, which are ignored in the ideal gas law.


Step 2: Key Formula or Approach:

The van der Waals equation for one mole of a real gas is:
\[ \left(P + \frac{a}{V^2}\right) (V - b) = RT \]
We need to understand the physical significance of each correction term.


Step 3: Detailed Explanation:

The ideal gas law (\(PV=RT\)) assumes that gas particles have no volume and no intermolecular forces.
The van der Waals equation introduces two correction factors, 'a' and 'b', to account for the behavior of real gases.


Volume Correction (V - b): The term '\(b\)' represents the volume occupied by the gas molecules themselves.
It is subtracted from the container volume (\(V\)) to give the effective volume in which the molecules can move.
This term, \((V - b)\), corrects for the finite size of gas molecules.

Pressure Correction (\(P + a/V^2\)): The term '\(a/V^2\)' accounts for the intermolecular forces of attraction.
In a real gas, molecules attract each other, which reduces the force with which they hit the container walls.
This leads to the observed pressure (\(P\)) being lower than the ideal pressure.
To correct for this, a term is added to the observed pressure.
This correction term is \(a/V^2\), where 'a' is a constant that depends on the strength of the intermolecular forces.
The entire term \(\left(P + \frac{a}{V^2}\right)\) represents the corrected, ideal pressure.


Therefore, the term that accounts for intermolecular attraction is the pressure correction term.


Step 4: Final Answer:

The term \((P + a/V^2)\) in the van der Waals equation accounts for the intermolecular attraction forces.
Quick Tip: Remember the physical meaning of the van der Waals constants:
\textbf{`a`}: Accounts for \textbf{a}ttraction forces (pressure correction).
\textbf{`b`}: Accounts for molecular \textbf{b}igness (volume correction).

Step 1: Understanding the Question:

The question relates the densities of two gases to their molecular weights under the same conditions.


Step 2: Key Formula or Approach:

We can derive the relationship between density (\(d\)) and molecular weight (\(M_w\)) from the ideal gas law:
\[ PV = nRT \]
Since the number of moles is \[ n = \frac{mass (m)}{Molecular Weight (M_w)}, \]
we have \[ PV = \frac{m}{M_w} RT. \]

Rearranging the equation to obtain density, where \(d = \frac{m}{V}\), \[ P M_w = \frac{m}{V} RT, \]
or \[ P M_w = dRT. \]

Hence, \[ d = \frac{P M_w}{RT}. \]

This shows that at constant temperature (\(T\)) and pressure (\(P\)), the density of a gas is directly proportional to its molecular weight: \[ d \propto M_w. \]

Step 3: Detailed Explanation:

Let \(d_X\) and \(M_X\) be the density and molecular weight of gas X.

Let \(d_Y\) and \(M_Y\) be the density and molecular weight of gas Y.

From the proportionality \(d \propto M_w\), we can write the ratio:
\[ \frac{d_X}{d_Y} = \frac{M_X}{M_Y} \]
The problem states:


\(d_X = 2 \times d_Y\) (The density of gas 'X' is twice that of gas 'Y').

\(M_Y = M\) (The molecular weight of gas 'Y' is 'M').


Substituting these values into the ratio:
\[ \frac{2 \times d_Y}{d_Y} = \frac{M_X}{M} \] \[ 2 = \frac{M_X}{M} \]
Solving for \(M_X\):
\[ M_X = 2M \]

Step 4: Final Answer:

The molecular weight of gas 'X' will be 2M.
Quick Tip: A useful shortcut for gas comparisons: at the same T and P, the ratio of densities is equal to the ratio of molecular weights (\(d_1/d_2 = M_1/M_2\)).
This comes from Avogadro's Law, which states equal volumes of gases at the same T and P contain equal numbers of moles.

Step 1: Understanding the Question:

We have two gases, initially in separate volumes at different pressures, which are then mixed in a new, larger container.
We need to find the total pressure of the resulting mixture, assuming constant temperature.


Step 2: Key Formula or Approach:

We will use two fundamental gas laws:


Boyle's Law: For a fixed amount of gas at constant temperature, the pressure and volume are inversely proportional (\(P_1V_1 = P_2V_2\)).
We will use this to find the partial pressure of each gas in the final container.

Dalton's Law of Partial Pressures: The total pressure of a gas mixture is the sum of the partial pressures of the individual gases (\(P_{total} = P_1 + P_2 + ...\)).



Step 3: Detailed Explanation:

First, note the volumes:

Initial volume for each gas = 500 C.C = 0.5 L.

Final volume of the mixture = 1 litre = 1000 C.C.

Part 1: Calculate the final partial pressure of hydrogen (\(H_2\)).

Initial conditions for \(H_2\): \(P_{H2,1} = 700\) mm Hg, \(V_1 = 500\) C.C.

Final conditions for \(H_2\): \(P_{H2,2}\) = ?, \(V_2 = 1000\) C.C.

Using Boyle's Law: \(P_{H2,1}V_1 = P_{H2,2}V_2\)
\[ 700 mm Hg \times 500 C.C = P_{H2,2} \times 1000 C.C \] \[ P_{H2,2} = \frac{700 \times 500}{1000} = 350 mm Hg \]
Part 2: Calculate the final partial pressure of oxygen (\(O_2\)).

Initial conditions for \(O_2\): \(P_{O2,1} = 600\) mm Hg, \(V_1 = 500\) C.C.

Final conditions for \(O_2\): \(P_{O2,2}\) = ?, \(V_2 = 1000\) C.C.

Using Boyle's Law: \(P_{O2,1}V_1 = P_{O2,2}V_2\)
\[ 600 mm Hg \times 500 C.C = P_{O2,2} \times 1000 C.C \] \[ P_{O2,2} = \frac{600 \times 500}{1000} = 300 mm Hg \]
Part 3: Calculate the total final pressure.

Using Dalton's Law: \(P_{total} = P_{H2,2} + P_{O2,2}\)
\[ P_{total} = 350 mm Hg + 300 mm Hg = 650 mm Hg \]

Step 4: Final Answer:

The final pressure of the gas mixture will be 650 mm Hg.
Quick Tip: When gases are mixed, first calculate the new partial pressure for each gas in the total final volume using Boyle's Law.
Then, simply add up the partial pressures to get the total pressure.

Step 1: Understanding the Question:

We are mixing air (a mixture of \(O_2\) and \(N_2\)) with pure \(O_2\) to create a final mixture that is 50% \(O_2\) by volume.
We need to find the ratio of the moles of air to the moles of pure oxygen added.
It's important to note the phrasing of the answer.
The given answer 0.58 corresponds to the ratio of "oxygen used" to "moles of air".
Let's solve for that ratio.


Step 2: Key Formula or Approach:

We will perform a mole balance.
For ideal gases, volume percent is equal to mole percent.
We will assume air is 21% \(O_2\) and 79% \(N_2\).

Let's define our variables:


\(A\) = moles of air fed to the mixer.

\(O\) = moles of pure oxygen fed to the mixer.

\(P\) = moles of the final product (enriched air).



Step 3: Detailed Explanation:

We can write mole balances for each component.
Let's assume a basis of \(P = 100\) moles of product.

Product Composition (Basis: 100 moles):


Moles of \(O_2\) in product = 50% of 100 = 50 moles.

Moles of \(N_2\) in product = 50% of 100 = 50 moles.



Nitrogen Balance:

All the nitrogen in the product must come from the air.

Moles of \(N_2\) in product = Moles of \(N_2\) in air feed.
\[ 50 = 0.79 \times A \]
Solving for A (moles of air):
\[ A = \frac{50}{0.79} \approx 63.29 moles of air \]
Oxygen Balance:

The oxygen in the product comes from two sources: the air and the pure oxygen stream.

Moles of \(O_2\) in product = (Moles of \(O_2\) from air) + (Moles of pure \(O_2\) added).
\[ 50 = (0.21 \times A) + O \]
We already found \(A = 63.29\) moles.
\[ 50 = (0.21 \times 63.29) + O \] \[ 50 = 13.29 + O \]
Solving for O (moles of pure oxygen):
\[ O = 50 - 13.29 = 36.71 moles of pure oxygen \]
Calculate the Ratio:

The question asks for "the ratio of moles of air to oxygen used".
A direct calculation gives \(A/O = 63.29 / 36.71 \approx 1.72\), which is option (A).
However, the provided answer key indicates (B) 0.58. This value is obtained by calculating the inverse ratio of "moles of oxygen used to moles of air".
\[ Ratio as per key = \frac{O}{A} = \frac{36.71}{63.29} \approx 0.5799 \approx 0.58 \]
We follow the key, noting the question's wording is ambiguous or mismatched with the intended answer.


Step 4: Final Answer:

The ratio of moles of pure oxygen used to moles of air is approximately 0.58.
Quick Tip: In material balance problems, always start by defining a basis (e.g., 100 moles of product).
Then, perform a balance on a tie component (a substance that appears in only one input and one output stream), like Nitrogen in this case.
Be careful with the wording of ratios, as "A to B" is the inverse of "B to A".

Step 1: Understanding the Question:

We need to calculate the density of carbon dioxide (\(CO_2\)) gas in Kg/m\(^3\) under specified conditions of temperature and pressure, assuming it behaves as an ideal gas.


Step 2: Key Formula or Approach

The ideal gas law can be rearranged to solve for density (\(d\)).
\[ PV = nRT \]

Since \[ n = \frac{mass (m)}{Molar Mass (M_w)}, \]
the equation becomes \[ PV = \frac{m}{M_w} RT. \]

Rearranging for density, where \(d = \frac{m}{V}\), \[ d = \frac{P M_w}{RT}. \]

\medskip
Step 3: Detailed Explanation

Part 1: Identify the values and convert units.


Pressure (\(P\)) = \(2\,atm\).

Temperature (\(T\)) = \(263^\circC\).
Converting to Kelvin:
\[ T = 263 + 273.15 = 536.15\,K. \]

Molar mass of \(\mathrm{CO_2}\) (\(M_w\)):
Carbon = \(12.01\,g mol^{-1}\),
Oxygen = \(16.00\,g mol^{-1}\).
\[ M_w = 12.01 + 2 \times 16.00 = 44.01\,g mol^{-1}. \]

Ideal gas constant (\(R\)):
\[ R = 0.0821\,L atm mol^{-1}K^{-1}. \]



Part 2: Calculate the density.

Using the formula \(d = \frac{P M_w}{RT}\):
\[ d = \frac{(2 atm) \times (44.01 g/mol)}{(0.0821 \frac{L \cdot atm}{mol \cdot K}) \times (536.15 K)} \] \[ d = \frac{88.02}{44.0179} g/L \] \[ d \approx 1.9996 g/L \]
Part 3: Convert density to final units.

The question asks for the density in Kg/m\(^3\).

We know that 1 g = \(10^{-3}\) Kg and 1 L = \(10^{-3}\) m\(^3\).

Therefore, 1 g/L = \(\frac{10^{-3} Kg}{10^{-3} m^3} = 1\) Kg/m\(^3\).

So, the density is approximately 2.0 Kg/m\(^3\).


Step 4: Final Answer:

The density of carbon dioxide at the given conditions is approximately 2 Kg/m\(^3\).
Quick Tip: When using the ideal gas law, unit consistency is critical.
Always convert temperature to Kelvin.
Choose the value of R that matches your other units (e.g., use R=8.314 J/(mol·K) if pressure is in Pascals and volume in m\(^3\)).
Remember that 1 g/L is numerically equal to 1 kg/m\(^3\).

Step 1: Understanding the Question:

We have a two-component solution (A and B) with a known overall specific gravity and composition by weight.
We know the specific gravity of component A and need to find the specific gravity of component B.


Step 2: Key Formula or Approach:

For an ideal mixture, the total volume is the sum of the individual component volumes (assuming volume additivity).
The relationship between density (\(\rho\)), mass (\(m\)), and volume (\(V\)) is \(\rho = m/V\).
Specific Gravity (SG) is the ratio of the substance's density to the density of water.
For simplicity in calculations using g and cm\(^3\), we can treat SG as being numerically equal to density in g/cm\(^3\) (since \(\rho_{water} \approx 1\) g/cm\(^3\)).

The governing equation is based on volume additivity: \(V_{mix} = V_A + V_B\).

This can be written in terms of mass and density:
\[ \frac{m_{mix}}{\rho_{mix}} = \frac{m_A}{\rho_A} + \frac{m_B}{\rho_B} \]
Dividing by \(m_{mix}\) gives the formula in terms of mass fractions (\(w\)):
\[ \frac{1}{\rho_{mix}} = \frac{w_A}{\rho_A} + \frac{w_B}{\rho_B} \]

Step 3: Detailed Explanation:

Let's list the given information:


SG\(_{mix}\) = 1.0 \(\implies \rho_{mix} = 1.0\) g/cm\(^3\).

Mass fraction of A (\(w_A\)) = 35% = 0.35.

Mass fraction of B (\(w_B\)) = 100% - 35% = 65% = 0.65.

SG\(_A\) = 0.7 \(\implies \rho_A = 0.7\) g/cm\(^3\).

We need to find \(\rho_B\).



A direct calculation using the standard formula for ideal mixtures yields:
\[ \frac{1}{1.0} = \frac{0.35}{0.7} + \frac{0.65}{\rho_B} \] \[ 1 = 0.5 + \frac{0.65}{\rho_B} \] \[ 1 - 0.5 = \frac{0.65}{\rho_B} \] \[ 0.5 = \frac{0.65}{\rho_B} \] \[ \rho_B = \frac{0.65}{0.5} = 1.3 g/cm^3 \]
This calculation gives a result of 1.3, which corresponds to option (B).
However, the provided answer key indicates that the correct answer is (D) 1.2.
This discrepancy suggests either a typographical error in the problem data or that the solution does not behave ideally.
For the answer to be 1.2, the specific gravity of A would need to be approximately 0.76 instead of 0.7.
Following the provided answer key, we select 1.2.


Step 4: Final Answer:

Based on the provided answer key, the specific gravity of B is 1.2.
This result implies an error in the question's premise or assumes non-ideal mixing.
Quick Tip: The formula \(\frac{1}{\rho_{mix}} = \sum \frac{w_i}{\rho_i}\) is essential for solving ideal mixture density problems based on mass fractions.
Always double-check if your calculated answer matches the options, as discrepancies can point to errors in the question or non-ideal behavior.

Step 1: Understanding the Question:

The question asks for the percentage humidity, given the relative humidity, temperature, total pressure, and the saturation vapor pressure of water.


Step 2: Key Formula or Approach:

The definitions are as follows:


Relative Humidity (\(H_R\)): The ratio of the partial pressure of water vapor (\(p_w\)) to the saturation vapor pressure of water (\(p_{ws}\)) at the same temperature.

\[ H_R = \frac{p_w}{p_{ws}} \times 100% \]
Percentage Humidity (\(H_P\)): The ratio of the actual mass of water vapor per unit mass of dry air to the mass of water vapor that would exist if the air were saturated at the same temperature and total pressure.
It is calculated using partial pressures as:

\[ H_P = \frac{p_w / (P - p_w)}{p_{ws} / (P - p_{ws})} \times 100% \]


Step 3: Detailed Explanation:

Part 1: Find the partial pressure of water vapor (\(p_w\)).

Given: \(H_R = 80%\), \(p_{ws} = 17.5\) mm Hg.
\[ 80% = \frac{p_w}{17.5 mm Hg} \times 100% \] \[ p_w = 0.80 \times 17.5 = 14.0 mm Hg \]
Part 2: Calculate the Percentage Humidity (\(H_P\)).

Given: Total pressure \(P = 750\) mm Hg.

We have \(p_w = 14.0\) mm Hg and \(p_{ws} = 17.5\) mm Hg.
\[ H_P = \frac{14 / (750 - 14)}{17.5 / (750 - 17.5)} \times 100% \] \[ H_P = \frac{14 / 736}{17.5 / 732.5} \times 100% \] \[ H_P = \frac{0.0190217}{0.0238908} \times 100% \] \[ H_P = 0.79619 \times 100% = 79.62% \]
The standard calculation yields a value of 79.62%, which is option (C).
The provided answer key indicates that (A) 88 is the correct answer.
This is a significant discrepancy that cannot be reconciled using standard definitions and the provided data.
To obtain an answer of 88, the initial data (e.g., total pressure) would need to be different.
Following the instruction to adhere to the answer key, we select 88.


Step 4: Final Answer:

Based on the provided answer key, the percentage humidity is 88, though a rigorous calculation with the given values yields 79.62%.
Quick Tip: Be clear on the difference between Relative Humidity (compares partial pressures of water) and Percentage Humidity (compares mass ratios of water to dry air).
The formula for percentage humidity includes the total pressure P, while the formula for relative humidity does not.
Discrepancies between calculations and answer keys often point to errors in the source material.

Step 1: Understanding the Question:

The question asks for a process that can reduce rancidity in oils.


Step 2: Detailed Explanation:

Rancidity is the process of oxidation or hydrolysis of fats and oils, leading to an unpleasant odor and flavor.
The primary cause of oxidative rancidity is the reaction of atmospheric oxygen with the unsaturated fatty acid chains in the oil.
These reactions often occur at the carbon-carbon double bonds.

Let's analyze the given processes:


Hydrogenation: This is a chemical process where hydrogen gas (\(H_2\)) is added across the carbon-carbon double bonds in unsaturated fats, converting them into single bonds (saturating them).
Saturated fats are much more stable and far less susceptible to oxidation.
Therefore, hydrogenation directly prevents the primary cause of oxidative rancidity, increasing the stability and shelf life of the oil.
This is the process used to make margarine from vegetable oil.

Oxidation: This is the very process that *causes* rancidity, so it would not reduce it.

Decolouration: This process, also known as bleaching, removes color pigments from the oil but does not significantly affect its susceptibility to rancidity.

Winterization: This is a process to remove fats with higher melting points from the oil so that the oil remains clear at low temperatures.
It does not prevent rancidity.



Step 3: Final Answer:

Hydrogenation reduces rancidity by converting unstable unsaturated fats into more stable saturated fats.
Quick Tip: Connect the chemical structure to stability.
Unsaturated fats (with C=C double bonds) are more reactive and prone to oxidation (rancidity).
Hydrogenation removes these double bonds, increasing stability.

Step 1: Understanding the Question:

The question asks for the chemical composition of the "cooking liquor" used in the sulphite process of papermaking.


Step 2: Detailed Explanation:

The goal of chemical pulping is to dissolve the lignin that binds cellulose fibers together in wood, without significantly degrading the cellulose fibers themselves.
There are two main chemical pulping processes:


Sulphite Process: This process uses an acidic cooking liquor.
The active chemicals are generated by dissolving sulfur dioxide (\(SO_2\)) in an aqueous solution of a base.
This creates a solution containing bisulphite (\(HSO_3^-\)) ions.
Depending on the pH, sulphite (\(SO_3^{2-}\)) ions are also present in equilibrium.
The base can be calcium, magnesium, sodium, or ammonium.
When sodium is the base, the active components are sodium bisulphite (\(NaHSO_3\)) and sodium sulphite (\(Na_2SO_3\)).

Kraft (or Sulphate) Process: This is an alkaline process and is more common today.
Its cooking liquor ("white liquor") contains sodium hydroxide (\(NaOH\)) and sodium sulphide (\(Na_2S\)).



The question specifically asks about the sulphite process.
Option (A) describes the Kraft process liquor.
Options (B) and (D) mention magnesium, which can be used as a base, but they incorrectly list sulphides instead of sulphites/bisulphites or are incomplete.
Option (C) correctly identifies the key chemical species, sodium sulphite and sodium bisulphite, for a sodium-based sulphite process.


Step 3: Final Answer:

The cooking liquor in the sulphite process consists of sulphite and bisulphite salts, such as sodium sulphite and sodium bisulphite.
Quick Tip: Remember the key chemical difference between the two main pulping processes:
\textbf{Sulphite Process} = Acidic, uses bisulphites (\(HSO_3^-\)).
\textbf{Kraft (Sulphate) Process} = Alkaline, uses sulphide (\(S^{2-}\)) and hydroxide (\(OH^-\)).

Step 1: Understanding the Question:

The question asks to identify the type of coal with the highest calorific value, which is the amount of heat produced during its complete combustion.


Step 2: Detailed Explanation:

Coal is classified into different "ranks" based on the degree of transformation (metamorphism) it has undergone from its original plant matter state (peat).
This ranking process is called coalification.
As the rank of coal increases, its carbon content increases, while moisture and volatile matter decrease.

The general order of coal ranks from lowest to highest is:

Peat \(\rightarrow\) Lignite \(\rightarrow\) Sub-bituminous \(\rightarrow\) Bituminous \(\rightarrow\) Anthracite

The calorific value is directly related to the carbon content.
Higher carbon content means more energy is released upon combustion.


Peat: Not technically coal, it is the precursor.
It has very high moisture and low carbon content, hence a low calorific value.

Lignite (Brown Coal): The lowest rank of coal.
It has low carbon content (60-70%) and high moisture content, resulting in a low calorific value.

Sub-bituminous Coal: Has characteristics between lignite and bituminous coal.

Bituminous Coal: A widely used type of coal with good calorific value and a carbon content of 70-86%.

Anthracite: The highest rank of coal.
It is a hard, glossy black coal with the highest carbon content (\(>\)86%), the lowest moisture, and the lowest volatile matter.
Consequently, it has the highest calorific value.



Step 3: Final Answer:

Anthracite, being the highest rank of coal with the highest percentage of carbon, has the highest calorific value.
Quick Tip: Memorize the ranks of coal in order: Peat, Lignite, Sub-bituminous, Bituminous, Anthracite (PL S-B A).
As you move up the rank, carbon content and calorific value increase, while moisture and volatile content decrease.

Step 1: Understanding the Question:

The question asks what components are determined by the "Ultimate Analysis" of coal.


Step 2: Detailed Explanation:

There are two main standard methods for analyzing coal:


Proximate Analysis: This method determines the bulk components of coal as they relate to its combustion properties.
It measures four things:


Moisture

Volatile Matter (gases driven off when heated)

Fixed Carbon (the solid combustible residue left after volatiles are removed)

Ash (the non-combustible inorganic residue left after complete combustion)


Option (B) is a mix of proximate and ultimate analysis components.

Ultimate Analysis: This method determines the elemental composition of the coal.
It provides the mass percentage of the individual elements that make up the coal.
The primary elements determined are:


Carbon (C)

Hydrogen (H)

Nitrogen (N)

Sulphur (S)


The Ash content is also determined as part of this analysis, and Oxygen (O) is typically calculated by difference: O% = 100% - (C% + H% + N% + S% + Ash%).



Comparing this with the options, option (D) lists the core elements determined in an ultimate analysis (Carbon, hydrogen, nitrogen and sulphur).
While oxygen and ash are also part of the full report, this option is the most accurate description among the choices.


Step 3: Final Answer:

Ultimate analysis of coal determines the elemental composition, primarily carbon, hydrogen, nitrogen, and sulphur.
Quick Tip: To avoid confusion, remember:
\textbf{Proximate Analysis} = \textbf{Properties} (Moisture, Volatiles, Fixed Carbon, Ash).
\textbf{Ultimate Analysis} = \textbf{Elements} (C, H, N, S, O, Ash).

Step 1: Understanding the Question:

The question asks for the catalyst used in a "shift converter".
This refers to the water-gas shift (WGS) reaction.
\[ CO + H_2O \rightleftharpoons CO_2 + H_2 \]

Step 2: Detailed Explanation:

The water-gas shift reaction is crucial in industrial processes like ammonia synthesis and hydrogen production to increase the hydrogen yield and remove carbon monoxide.
The process is typically carried out in two stages:


High-Temperature Shift (HTS) Converter: Operates at \(350-450^\circ\)C using an iron oxide-chromium oxide (\(Fe_2O_3-Cr_2O_3\)) catalyst.

Low-Temperature Shift (LTS) Converter: Operates at \(200-250^\circ\)C using a copper-zinc oxide-alumina (\(Cu-ZnO-Al_2O_3\)) catalyst.


None of the standard catalysts are listed as options.
We must evaluate the given choices:


Nickel (Ni): Nickel is a primary catalyst for steam reforming (e.g., \(CH_4 + H_2O \rightarrow CO + 3H_2\)), which produces the syngas that is then fed to the shift converter.
While performing reforming, the nickel catalyst also catalyzes the water-gas shift reaction to some extent, helping the gas mixture approach equilibrium.
In this broader context of syngas processing, nickel is a plausible, though not a specific, shift converter catalyst.

Vanadium: Vanadium pentoxide (\(V_2O_5\)) is the catalyst for the Contact Process (\(SO_2\) to \(SO_3\)) for sulfuric acid production.

Silica gel and Alumina: These are often used as catalyst supports due to their high surface area, but they are not the primary catalytic agents for the WGS reaction.


Given the options, Nickel is the most closely related and catalytically active metal for the overall process in which shift conversion is a key step.
It is the most likely intended answer.


Step 3: Final Answer:

Among the given choices, Nickel is the most appropriate answer as it is a key catalyst in syngas production and can also catalyze the shift reaction itself.
Quick Tip: For catalyst questions, know the specific catalysts for major industrial processes:
Iron/Chromium (HTS), Copper/Zinc (LTS), Nickel (Reforming/Hydrogenation), Vanadium Pentoxide (Contact Process), and Iron (Haber-Bosch process).

Step 1: Understanding the Question:

The question asks for the common or trade name of the polymer Polytetrafluoroethylene.


Step 2: Detailed Explanation:

This is a direct knowledge question about common polymers.


Polytetrafluoroethylene (PTFE): This is a synthetic fluoropolymer of tetrafluoroethylene.
It is well-known for its non-stick properties, high thermal stability, and chemical resistance.
Its most famous trade name, registered by DuPont and now owned by Chemours, is Teflon.

Nylon: This is a generic name for a family of synthetic polyamides.
It is known for its use in textiles, carpets, and molded parts.

Bakelite: This is a trade name for a thermosetting phenol formaldehyde resin.
It was one of the first synthetic plastics and is known for its electrical non-conductivity and heat-resistant properties.

Rayon: This is a semi-synthetic fiber made from regenerated cellulose (from wood pulp).
It is used in textiles.


Therefore, the common name for polytetrafluoroethylene is Teflon.


Step 3: Final Answer:

Poly tetra fluoro ethylene is known as Teflon.
Quick Tip: It is very helpful to memorize the chemical names and common trade names of major polymers for chemistry and materials science exams.
Examples: PTFE (Teflon), Polycarbonate (Lexan), Polymethyl methacrylate (Plexiglas/Lucite).

Step 1: Understanding the Question:

The question asks for the definition of the "hydrophilic" part of a soap or detergent molecule.


Step 2: Detailed Explanation:

Soap and detergent molecules are surfactants (surface-active agents).
They have a characteristic structure with two distinct parts:


A Hydrophobic Tail: This is a long, nonpolar hydrocarbon chain.
The term "hydrophobic" literally means "water-fearing" or water hating.
This part of the molecule avoids water and prefers to dissolve in nonpolar substances like oil and grease (dirt).

A Hydrophilic Head: This is a polar or ionic group at one end of the molecule (e.g., a carboxylate group \(-COO^-Na^+\) in soap).
The term "hydrophilic" literally means "water-loving".
This polar head is attracted to polar water molecules and allows the surfactant to dissolve in water.


This dual nature allows soaps and detergents to act as emulsifying agents, surrounding droplets of oil (with their hydrophobic tails) and allowing them to be washed away in water (thanks to their hydrophilic heads).

The term "soil loving" is synonymous with "lipophilic" (oil-loving), which describes the hydrophobic tail, not the hydrophilic head.


Step 3: Final Answer:

The hydrophilic group of a soap or detergent is the water-loving part of the molecule.
Quick Tip: Break down scientific terms into their Greek/Latin roots:
\textbf{Hydro} = Water
\textbf{Phobic} = Fearing
\textbf{Philic} = Loving
\textbf{Lipo} = Fat/Oil
This makes terms like hydrophobic, hydrophilic, and lipophilic easy to remember.

Step 1: Understanding the Question:

The question asks to identify the chemical agent used in the final stages of pulp bleaching to achieve high brightness by removing residual color.


Step 2: Detailed Explanation:

Pulp bleaching is a multi-stage process designed to remove lignin, the natural polymer that imparts a brown color to wood pulp. The final stages, often called "end bleaching" or brightness stages, aim to remove the last traces of color without damaging the cellulose fibers.

Let's analyze the options:


(A) \(ClO_2\) (Chlorine Dioxide): This is a powerful and selective oxidizing agent. It is highly effective at breaking down residual lignin and other color-causing compounds without significantly degrading cellulose strength. It is the most common bleaching agent used in the final stages of modern Elemental Chlorine Free (ECF) bleaching sequences to achieve high brightness levels.

(B) MgO (Magnesium Oxide): This is not a bleaching agent. It is sometimes used in pulp processing, for example, to control pH or as a base in magnesium-based pulping.

(C) \(SO_2\) gas (Sulfur Dioxide): This is a reducing agent and can be used for bleaching, particularly in mechanical pulps. However, in chemical pulps, it is more commonly used as an "antichlor" to remove residual chlorine-based bleaching agents from previous stages, rather than as the primary end bleaching agent for high brightness.

(D) Mercaptans: These are organosulfur compounds known for their strong, unpleasant odors. They are byproducts of the Kraft pulping process and must be removed, not used as bleaching agents.



Step 3: Final Answer:

Chlorine dioxide (\(ClO_2\)) is the most effective and widely used end bleaching agent to remove the final traces of color from pulp.
Quick Tip: In pulp bleaching, remember the difference between bulk delignification (removing most lignin, e.g., with oxygen) and final bleaching (achieving brightness, e.g., with \(ClO_2\)). Modern processes favor agents like \(ClO_2\) and ozone over elemental chlorine (\(Cl_2\)) for environmental reasons.

Step 1: Understanding the Question:

The question asks for the chemical composition of the material known as Carborundum.


Step 2: Detailed Explanation:

This is a direct knowledge question about an industrial material.


Carborundum is a well-known trade name for Silicon Carbide (SiC). It is a synthetic compound of silicon and carbon.

Silicon carbide is an extremely hard ceramic material (about 9 on the Mohs scale, close to diamond). Because of its hardness and abrasive properties, it is widely used in grinding wheels, sandpaper, cutting tools, and high-performance brake discs.

Bauxite is an aluminum ore, primarily composed of aluminum oxides and hydroxides (\(Al_2O_3 \cdot nH_2O\)).

Boron Carbide (\(B_4C\)) is another extremely hard ceramic, even harder than silicon carbide, but it is not called Carborundum.

Calcium Carbide (\(CaC_2\)) is a chemical compound primarily used to produce acetylene gas (\(C_2H_2\)) upon reaction with water.



Step 3: Final Answer:

Carborundum is the common name for Silicon Carbide (SiC).
Quick Tip: Memorizing common or trade names of important industrial chemicals is useful. Carborundum = Silicon Carbide (SiC) is a classic example.

Step 1: Understanding the Question:

The question asks to identify the unsaturated fatty acid from the given options.


Step 2: Detailed Explanation:

A fatty acid consists of a long hydrocarbon chain with a carboxyl group at one end. The key distinction is:


Saturated fatty acids: The hydrocarbon chain contains only carbon-carbon single bonds.

Unsaturated fatty acids: The hydrocarbon chain contains one or more carbon-carbon double bonds (C=C).


Let's analyze the options:


(A) Palmitic acid (\(C_{16}H_{32}O_2\)): A 16-carbon fatty acid with no double bonds. It is saturated.

(B) Stearic acid (\(C_{18}H_{36}O_2\)): An 18-carbon fatty acid with no double bonds. It is saturated.

(C) Oxalic acid (\(C_2H_2O_4\)): This is a dicarboxylic acid and is not classified as a fatty acid.

(D) Oleic acid (\(C_{18}H_{34}O_2\)): An 18-carbon fatty acid that has one carbon-carbon double bond. It is a monounsaturated fatty acid.



Step 3: Final Answer:

Oleic acid is an unsaturated fatty acid because its hydrocarbon chain contains a double bond.
Quick Tip: For fatty acids, you can use the general formula \(C_nH_{2n}O_2\) for saturated acids. If the number of hydrogens is less than \(2n\), the acid is unsaturated. For oleic acid (\(C_{18}\)), a saturated acid would have \(H_{36}\), but it only has \(H_{34}\), indicating one double bond.

Step 1: Understanding the Question:

The question asks for the commercial process used to produce ethylene from naphtha.


Step 2: Detailed Explanation:

The primary industrial method for producing light olefins like ethylene and propylene from hydrocarbon feedstocks such as naphtha is Steam Cracking.

Steam Cracking is a process that takes place at very high temperatures (typically \(750 - 950^\circ\)C) in the presence of steam. The high temperature causes the long-chain hydrocarbon molecules in naphtha to break apart into smaller, more valuable molecules. This process is a form of Pyrolysis (thermal decomposition in the absence of oxygen) and can also be classified as Thermal Cracking.


Let's evaluate the options based on standard terminology:


(B) Pyrolysis and (D) Thermal cracking are both technically correct descriptions of steam cracking.

(A) Catalytic cracking (specifically Fluid Catalytic Cracking, FCC) is primarily used to produce high-octane gasoline from heavier oil fractions. It does produce some ethylene, but it is not the main process for ethylene production.

(C) Hydrocracking is a cracking process carried out in the presence of a catalyst and a hydrogen-rich atmosphere. It is used to produce high-quality saturated products like gasoline and jet fuel. It is specifically designed to avoid the production of olefins like ethylene.



Step 3: Final Answer:

Based on standard chemical engineering principles, the most accurate answer would be Pyrolysis or Thermal Cracking. However, the provided answer key indicates that Hydrocracking is the correct choice. This is factually incorrect in standard industrial practice, as hydrocracking aims to produce saturated hydrocarbons, not olefins like ethylene. We select this answer to align with the provided key, acknowledging the significant technical discrepancy.
Quick Tip: For petrochemical questions, remember the primary goal of each cracking process: \textbf{Steam Cracking (Pyrolysis/Thermal): Produces olefins (ethylene, propylene). \textbf{Catalytic Cracking (FCC):} Produces gasoline. \textbf{Hydrocracking:} Produces high-quality saturated fuels (gasoline, jet fuel, diesel).

Step 1: Understanding the Question:

The question asks for the reactants used to manufacture the fertilizer known as Triple Superphosphate (TSP).


Step 2: Key Formula or Approach:

Phosphate fertilizers are produced by treating phosphate rock, which contains insoluble calcium phosphate (\(Ca_3(PO_4)_2\)), with acid to convert it into a water-soluble form that plants can absorb.


Step 3: Detailed Explanation:

Let's compare the production of different superphosphates:


Single Superphosphate (SSP): This is produced by reacting phosphate rock with sulfuric acid (\(H_2SO_4\)). The product is a mixture of monocalcium phosphate (\(Ca(H_2PO_4)_2\)) and calcium sulfate (\(CaSO_4\)). The calcium sulfate is an inert diluent. This corresponds to option (B).

Triple Superphosphate (TSP): To create a more concentrated fertilizer without the calcium sulfate diluent, phosphate rock is reacted with phosphoric acid (\(H_3PO_4\)). This reaction produces a much higher concentration of the active ingredient, monocalcium phosphate.

The simplified reaction is: \(Ca_3(PO_4)_2 + 4H_3PO_4 \rightarrow 3Ca(H_2PO_4)_2\).

This corresponds to option (A).


Reacting phosphate rock with nitric acid produces nitrophosphate fertilizers. Ammonium phosphate is already a fertilizer product, not a raw material for TSP.


Step 4: Final Answer:

Triple superphosphate is manufactured by reacting phosphate rock with phosphoric acid.
Quick Tip: Remember the key difference: \textbf{Single} Superphosphate = Phosphate Rock + \textbf{Sulfuric} Acid. \textbf{Triple} Superphosphate = Phosphate Rock + \textbf{Phosphoric} Acid. The name "triple" refers to the roughly three times higher phosphorus content compared to SSP.

Step 1: Understanding the Question:

The question asks for the correct sequence of major unit operations in the Contact Process for manufacturing sulfuric acid from elemental sulfur.


Step 2: Detailed Explanation:

The Contact Process involves three main chemical steps, each corresponding to a major piece of equipment:


Step 1: Production of Sulfur Dioxide (\(SO_2\))

Molten elemental sulfur is sprayed into a FURNACE and burned in an excess of dry air to produce sulfur dioxide gas.

\[ S(l) + O_2(g) \rightarrow SO_2(g) \]
Step 2: Catalytic Oxidation to Sulfur Trioxide (\(SO_3\))

The hot \(SO_2\) gas is then passed through a multi-stage catalytic CONVERTER containing a vanadium pentoxide (\(V_2O_5\)) catalyst. Here, it is oxidized to sulfur trioxide.

\[ 2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \]
Step 3: Absorption of Sulfur Trioxide

The \(SO_3\) gas is then fed into an ABSORBER (or absorption tower), where it is absorbed into concentrated sulfuric acid (98-99%) to form oleum (\(H_2S_2O_7\)). The oleum is then diluted with water to produce sulfuric acid of the desired concentration.

\[ SO_3(g) + H_2SO_4(l) \rightarrow H_2S_2O_7(l) \]


Step 3: Final Answer:

The correct sequence of operations is: burning in the Furnace, followed by oxidation in the Converter, and finally absorption in the Absorber.
Quick Tip: Remember the flow of the Contact Process: Burn Sulfur (Furnace) \(\rightarrow\) Convert \(SO_2\) to \(SO_3\) (Converter) \(\rightarrow\) Absorb \(SO_3\) (Absorber). The name "Contact" process comes from the fact that the gases must come into contact with the solid catalyst in the converter.

Step 1: Understanding the Question:

The question asks for the meaning and application of the term BOD in the process industry.


Step 2: Detailed Explanation:

BOD stands for Biochemical Oxygen Demand (or sometimes Biological Oxygen Demand).


Definition: BOD is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose the organic matter present in a sample of water over a specific period (typically 5 days at \(20^\circ\)C, denoted as \(BOD_5\)).

Application: It is a primary indicator of the level of organic pollution in water. A high BOD value means there is a large amount of biodegradable organic material, which will consume significant amounts of oxygen from the water as it decomposes, potentially harming aquatic life.

Context: Therefore, BOD is used to measure the pollution load and characterize liquid effluents, such as wastewater from industrial plants or municipal sewage, to ensure they meet environmental discharge regulations.


It is not used for solid wastes, gaseous effluents, or typically for the purity of boiler feed water (which is characterized by measures like conductivity, hardness, and dissolved solids).


Step 3: Final Answer:

The term BOD signifies the characterisation of liquid effluents.
Quick Tip: Remember the key environmental metrics for different waste streams: \textbf{Liquid Effluents:} BOD, COD (Chemical Oxygen Demand), TSS (Total Suspended Solids). \textbf{Gaseous Effluents:} SOx, NOx, Particulate Matter. \textbf{Boiler Water:} Hardness, pH, Conductivity, Dissolved Oxygen.

Step 1: Understanding the Question:

The question asks for the primary feedstock used in the modern, large-scale industrial production of methanol.


Step 2: Detailed Explanation:

The vast majority of global methanol production is based on a catalytic process using Synthesis Gas (also known as syngas).


Synthesis Gas is a mixture of carbon monoxide (CO) and hydrogen (\(H_2\)). It is typically produced from natural gas (methane), coal, or biomass through processes like steam reforming or gasification.

The syngas is then passed over a catalyst (commonly a mixture of copper and zinc oxides) under high pressure and temperature to produce methanol (\(CH_3OH\)).

The main reactions are:

\[ CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g) \]
\[ CO_2(g) + 3H_2(g) \rightleftharpoons CH_3OH(g) + H_2O(g) \]

Let's analyze the other options:


(A) Formaldehyde: Formaldehyde is actually a major product made *from* methanol through oxidation.

(B) Acetic acid and (D) Ethanol are other important industrial chemicals but are not used as raw materials for methanol production.



Step 3: Final Answer:

The key raw material for the commercial production of methanol is Synthesis gas.
Quick Tip: Remember the central role of Synthesis Gas (CO + \(H_2\)) in industrial chemistry. It is a fundamental building block for many chemicals, including methanol and ammonia (via hydrogen), and for liquid fuels via the Fischer-Tropsch process.