Question

Find the inverse of the matrices , if it exists.
$\left[\begin{array}{cc}1& 3\\ 2& 7\end{array}\right]$

Answer 1

Let $A=\left[\begin{array}{cc}1& 3\\ 2& 7\end{array}\right]$
We know that $A=IA$

$\therefore A=\left[\begin{array}{cc}1& 3\\ 2& 7\end{array}\right]=\left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]A$

$\Rightarrow \left[\begin{array}{cc}1& 3\\ 0& 1\end{array}\right]=\left[\begin{array}{cc}1& 0\\ -2& 1\end{array}\right]A(R_2\to R_2-{2R}_1)$

$\Rightarrow \left[\begin{array}{cc}1& 0\\ 0& 1\end{array}\right]=\left[\begin{array}{cc}7& -3\\ -2& 1\end{array}\right]A(R_1\to R_1-3R_2)$

$\therefore A^{-1}=\left[\begin{array}{cc}7& -3\\ -2& 1\end{array}\right]$