Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 8 - Section 8.4 - Multiplicative Inverses of Matrices and Matrix Equations - Exercise Set - Page 933: 46

Answer

The inverse is $=\left[ \begin{matrix} \frac{1}{4} & \frac{5}{8} \\ \frac{1}{2} & \frac{3}{4} \\ \end{matrix} \right]$

Work Step by Step

Consider the matrix $ A=\left[ \begin{matrix} 7 & -5 \\ -4 & 3 \\ \end{matrix} \right]$. And, identity matrix is $ I=\left[ \begin{matrix} 1 & 0 \\ 0 & 1 \\ \end{matrix} \right]$ Then, $\begin{align} & \left[ I-A \right]=\left[ \begin{matrix} 1 & 0 \\ 0 & 1 \\ \end{matrix} \right]-\left[ \begin{matrix} 7 & -5 \\ -4 & 3 \\ \end{matrix} \right] \\ & =\left[ \begin{matrix} 1-7 & 0+5 \\ 0+4 & 1-3 \\ \end{matrix} \right] \\ & =\left[ \begin{matrix} -6 & 5 \\ 4 & -2 \\ \end{matrix} \right] \end{align}$ Now, the inverse of matrix $\left[ I-A \right]$ is equal to ${{A}^{-1}}=\frac{1}{ad-bc}\left[ \begin{matrix} d & -c \\ -d & a \\ \end{matrix} \right]$. Now, we will compare the matrix to the original matrix, $\begin{align} & a=-6 \\ & b=5 \\ & c=4 \\ & d=-2 \end{align}$ Now, the inverse is, ${{A}^{-1}}=\frac{1}{ad-bc}\left[ \begin{matrix} d & -c \\ -b & a \\ \end{matrix} \right]$ Substitute the values to get, $\begin{align} & {{A}^{-1}}=\frac{1}{ad-bc}\left[ \begin{matrix} d & -c \\ -b & a \\ \end{matrix} \right] \\ & =\frac{1}{-8}\left[ \begin{matrix} -2 & -5 \\ -4 & -6 \\ \end{matrix} \right] \\ & =\left[ \begin{matrix} \frac{1}{4} & \frac{5}{8} \\ \frac{1}{2} & \frac{3}{4} \\ \end{matrix} \right] \end{align}$ Therefore, the inverse of the matrix, ${{\left[ I-A \right]}^{-1}}=\left[ \begin{matrix} \frac{1}{4} & \frac{5}{8} \\ \frac{1}{2} & \frac{3}{4} \\ \end{matrix} \right]$.
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