Class 12 Maths

# Matrices

## NCERT Solution

### Exercise 2 Part 3

Question 4: If

 A = \begin{bmatrix}1 & 2 & -3\\5 & 0 & 2\\1 & -1 & 1\end{bmatrix}
 B = \begin{bmatrix}3 & -1 & 2\\4 & 2 & 5\\2 & 0 & 3\end{bmatrix}
 C = \begin{bmatrix}4 & 1 & 2\\0 & 3 & 2\\1 & -2 & 3\end{bmatrix}

Then compute (A + B) and (A – C). Also verify that A + (B - C) = (A + B) - C

Solution:

 A + B = \begin{bmatrix}4 & 1 & -1\\9 & 2 & 7\\3 & -1 & 4\end{bmatrix}
 B - C = \begin{bmatrix}-1 & -2 & 0\\4 & -1 & 3\\1 & 2 & 0\end{bmatrix}
 Now, A + (B – C) = \begin{bmatrix}0 & 0 & -3\\9 & -1 & 5\\2 & 1 & 1\end{bmatrix}
 And (A+B) – C = \begin{bmatrix}0 & 0 & -2\\9 & -1 & 5\\2 & 1 & 1\end{bmatrix}

Thus, A + (B - C) = (A + B) - C

Question 5: If

 A = \begin{bmatrix}2/3 & 1 & 5/3\\1/3 & 2/3 & 4/3\\7/3 & 2 & 2/3\end{bmatrix}
 B = \begin{bmatrix}2/5 & 3/5 & 1\\1/5 & 2/5 & 4/5\\7/5 & 6/5 & 2/5\end{bmatrix}

Compute 3A – 5B

Solution:

 3A = \begin{bmatrix}2 & 3 & 5\\1 & 2 & 4\\7 & 6 & 2\end{bmatrix}
 5B = \begin{bmatrix}2 & 3 & 5\\1 & 2 & 4\\7 & 6 & 2\end{bmatrix}
 Hence, 3A – B = \begin{bmatrix}0 & 0 & 0\\0 & 0 & 0\\0 & 0 & 0\end{bmatrix}

Question 6: Simplify

 text(cos)θ \begin{bmatrix}cosθ & sinθ\\-sinθ & cosθ\end{bmatrix} +text(sin)θ \begin{bmatrix}cosθ & -cosθ\\cosθ & sinθ\end{bmatrix}

Solution:

 = \begin{bmatrix}cos^2θ & sinθcosθ\\-sinθcosθ & cos^2θ\end{bmatrix} + \begin{bmatrix}sin^2θ & -sinθcosθ\\sinθcosθ & sin^2θ\end{bmatrix}
 = \begin{bmatrix}1 & 0\\0 & 1\end{bmatrix}