NCERT Solutions
Class 12 - Mathematics Part-1 - Chapter 3: MATRICES
Exercise 3.3

Question. 1

Find the transpose of each of the following matrices:

(i) \(\begin{bmatrix} 5 \\ \frac{1}{2} \\ -1 \end{bmatrix}\)

(ii) \(\begin{bmatrix} 1 & -1 \\ 2 & 3 \end{bmatrix}\)

(iii) \(\begin{bmatrix} -1 & 5 & 6 \\ \sqrt{3} & 5 & 6 \\ 2 & 3 & -1 \end{bmatrix}\)

Answer:

(i) \(\begin{bmatrix} 5 & \frac{1}{2} & -1 \end{bmatrix}\)

(ii) \(\begin{bmatrix} 1 & 2 \\ -1 & 3 \end{bmatrix}\)

(iii) \(\begin{bmatrix} -1 & \sqrt{3} & 2 \\ 5 & 5 & 3 \\ 6 & 6 & -1 \end{bmatrix}\)

Question. 2

If

\[ A = \begin{bmatrix} -1 & 2 & 3 \\ 5 & 7 & 9 \\ -2 & 1 & 1 \end{bmatrix} \quad \text{and} \quad B = \begin{bmatrix} -4 & 1 & -5 \\ 1 & 2 & 0 \\ 1 & 3 & 1 \end{bmatrix} \]

then verify that

(i) \((A + B)' = A' + B'\)

(ii) \((A - B)' = A' - B'\)

Answer:

Question. 3

If

\[ A' = \begin{bmatrix} 3 & 4 \\ -1 & 2 \\ 0 & 1 \end{bmatrix} \quad \text{and} \quad B = \begin{bmatrix} -1 & 2 & 1 \\ 1 & 2 & 3 \end{bmatrix} \]

then verify that

(i) \((A + B)' = A' + B'\)

(ii) \((A - B)' = A' - B'\)

Answer:

Question. 4

If

\[ A' = \begin{bmatrix} -2 & 3 \\ 1 & 2 \end{bmatrix} \quad \text{and} \quad B = \begin{bmatrix} -1 & 0 \\ 1 & 2 \end{bmatrix} \]

then find \((A + 2B)'\).

Answer:

\((A + 2B)' = \begin{bmatrix} -4 & 5 \\ 1 & 6 \end{bmatrix}\)

Question. 5

For the matrices \(A\) and \(B\), verify that \((AB)' = B'A'\), where

(i) \(A = \begin{bmatrix} 1 \\ -4 \\ 3 \end{bmatrix},\; B = \begin{bmatrix} -1 & 2 & 1 \end{bmatrix}\)

(ii) \(A = \begin{bmatrix} 0 \\ 1 \\ 2 \end{bmatrix},\; B = \begin{bmatrix} 1 & 5 & 7 \end{bmatrix}\)

Answer:

Question. 6

If

(i) \(A = \begin{bmatrix} \cos\alpha & \sin\alpha \\ -\sin\alpha & \cos\alpha \end{bmatrix}\), then verify that \(A'A = I\).

(ii) If \(A = \begin{bmatrix} \sin\alpha & \cos\alpha \\ -\cos\alpha & \sin\alpha \end{bmatrix}\), then verify that \(A'A = I\).

Answer:

Question. 7

(i) Show that the matrix

\[ A = \begin{bmatrix} 1 & -1 & 5 \\ -1 & 2 & 1 \\ 5 & 1 & 3 \end{bmatrix} \]

is a symmetric matrix.

(ii) Show that the matrix

\[ A = \begin{bmatrix} 0 & 1 & -1 \\ -1 & 0 & 1 \\ 1 & -1 & 0 \end{bmatrix} \]

is a skew symmetric matrix.

Answer:

Question. 8

For the matrix \(A = \begin{bmatrix} 1 & 5 \\ 6 & 7 \end{bmatrix}\), verify that

(i) \((A + A')\) is a symmetric matrix

(ii) \((A - A')\) is a skew symmetric matrix

Answer:

Question. 9

Find \(\frac{1}{2}(A + A')\) and \(\frac{1}{2}(A - A')\), when

\[ A = \begin{bmatrix} 0 & a & b \\ -a & 0 & c \\ -b & -c & 0 \end{bmatrix} \]

Answer:

\(\frac{1}{2}(A + A') = \begin{bmatrix} 0 & 0 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0 \end{bmatrix}\)

\(\frac{1}{2}(A - A') = \begin{bmatrix} 0 & a & b \\ -a & 0 & c \\ -b & -c & 0 \end{bmatrix}\)

Question. 10

Express the following matrices as the sum of a symmetric and a skew symmetric matrix:

(i) \(\begin{bmatrix} 3 & 5 \\ 1 & -1 \end{bmatrix}\)

(ii) \(\begin{bmatrix} 6 & -2 & 2 \\ -2 & 3 & -1 \\ 2 & -1 & 3 \end{bmatrix}\)

(iii) \(\begin{bmatrix} 3 & 3 & -1 \\ -2 & -2 & 1 \\ -4 & -5 & 2 \end{bmatrix}\)

(iv) \(\begin{bmatrix} 1 & 5 \\ -1 & 2 \end{bmatrix}\)

Answer:

(i) \(A = \begin{bmatrix} 3 & 3 \\ 3 & -1 \end{bmatrix} + \begin{bmatrix} 0 & 2 \\ -2 & 0 \end{bmatrix}\)

(ii) \(A = \begin{bmatrix} 6 & -2 & 2 \\ -2 & 3 & -1 \\ 2 & -1 & 3 \end{bmatrix} + \begin{bmatrix} 0 & 0 & 0 \\ 0 & 0 & 0 \\ 0 & 0 & 0 \end{bmatrix}\)

(iii) \(A = \begin{bmatrix} 3 & \frac{1}{2} & -\frac{5}{2} \\ \frac{1}{2} & -2 & -2 \\ -\frac{5}{2} & -2 & 2 \end{bmatrix} + \begin{bmatrix} 0 & \frac{5}{2} & \frac{3}{2} \\ -\frac{5}{2} & 0 & 3 \\ -\frac{3}{2} & -3 & 0 \end{bmatrix}\)

(iv) \(A = \begin{bmatrix} 1 & 2 \\ 2 & 2 \end{bmatrix} + \begin{bmatrix} 0 & 3 \\ -3 & 0 \end{bmatrix}\)

Question.  11

Choose the correct answer.

If \(A, B\) are symmetric matrices of same order, then \(AB - BA\) is a

(A)

Skew symmetric matrix

(B)

Symmetric matrix

(C)

Zero matrix

(D)

Identity matrix

Question.  12

Choose the correct answer.

If \(A = \begin{bmatrix} \cos\alpha & -\sin\alpha \\ \sin\alpha & \cos\alpha \end{bmatrix}\), and \(A + A' = I\), then the value of \(\alpha\) is

(A)

\(\frac{\pi}{6}\)

(B)

\(\frac{\pi}{3}\)

(C)

\(\pi\)

(D)

\(\frac{3\pi}{2}\)

NCERT Solutions Class 12 – Mathematics Part-1 – Chapter 3: MATRICES – Exercise 3.3 | Detailed Answers