Elementary Linear Algebra 7th Edition

Published by Cengage Learning
ISBN 10: 1-13311-087-8
ISBN 13: 978-1-13311-087-3

Chapter 2 - Matrices - 2.3 The Inverse of a Matrix - 2.3 Exercises: 2

Answer

$B=A^{-1}$

Work Step by Step

$A=\begin{pmatrix}1&-1\\-1&2\end{pmatrix}\quad B=\begin{pmatrix}2&1\\1&1\end{pmatrix}$ To verify that $B$ is the inverse of $A$ we simply need to perform matrix multiplication: $AB=\begin{pmatrix}1&-1\\-1&2\end{pmatrix}\begin{pmatrix}2&1\\1&1\end{pmatrix}=\begin{pmatrix}1\times2+(-1)\times1&1\times1+(-1)\times1\\(-1)\times2+2\times1&(-1)\times1+2\times1\end{pmatrix}$ $=\begin{pmatrix}1&0\\0&1\end{pmatrix}$ $BA=\begin{pmatrix}2&1\\1&1\end{pmatrix}\begin{pmatrix}1&-1\\-1&2\end{pmatrix}=\begin{pmatrix}2\times1+1\times(-1)&2\times(-1)+1\times2\\1\times1+1\times(-1)&1\times(-1)+1\times2\end{pmatrix}$ $=\begin{pmatrix}1&0\\0&1\end{pmatrix}$ and so $B=A^{-1}$
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