Linear Algebra: A Modern Introduction

Published by Cengage Learning
ISBN 10: 1285463242
ISBN 13: 978-1-28546-324-7

Chapter 3 - Matrices - 3.3 The Inverse of a Matrix - Exercises 3.3 - Page 178: 11

Answer

\[x=-5\;,\; y=9\]

Work Step by Step

\[2x+y=-1\\ 5x+3y=2\] Here $\;\;A=\left[\begin{array}{cc} 2&1\\ 5&3 \end{array}\right] \;\;,b=\left[\begin{array}{c} -1\\ 2\end{array}\right]$ And $\;\;X=\left[\begin{array}{c} x\\ y\end{array}\right]$ Consider $\;\;A=\left[\begin{array}{cc} 2&1\\ 5&3 \end{array}\right]$ Here $\;2(3)-1(5)=1\neq 0$ $\Rightarrow A$ is invertible $A^{-1}=\frac{1}{6-5}\left[\begin{array}{cc} 3&-1\\ -5&2 \end{array}\right]$ $\Rightarrow A^{-1}=\left[\begin{array}{cc} 3&-1\\ -5&2 \end{array}\right]$ $X=A^{-1}b$ $X=\left[\begin{array}{cc} 3&-1\\ -5&2 \end{array}\right]\left[\begin{array}{c} -1\\ 2\end{array}\right]$ $\Rightarrow X=\left[\begin{array}{c} -3-2\\ 5+4\end{array}\right]$ $\Rightarrow X=\left[\begin{array}{c} -5\\ 9\end{array}\right]$ $\Rightarrow \left[\begin{array}{c} x\\ y\end{array}\right] =\left[\begin{array}{c} -5\\ 9\end{array}\right]$ $\Rightarrow x=-5\:,\;y=9$ Hence $x=-5\:,\;y=9$.
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