Gauss-Jordan elimination on $$$\left[\begin{array}{c|c}a & -1\\b & 1\end{array}\right]$$$
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Perform the Gauss-Jordan elimination (reduce completely) on $$$\left[\begin{array}{cc}a & -1\\b & 1\end{array}\right]$$$.
Solution
Divide row $$$1$$$ by $$$a$$$: $$$R_{1} = \frac{R_{1}}{a}$$$.
$$$\left[\begin{array}{c|c}1 & - \frac{1}{a}\\b & 1\end{array}\right]$$$
Subtract row $$$1$$$ multiplied by $$$b$$$ from row $$$2$$$: $$$R_{2} = R_{2} - b R_{1}$$$.
$$$\left[\begin{array}{c|c}1 & - \frac{1}{a}\\0 & \frac{a + b}{a}\end{array}\right]$$$
Answer
The reduced matrix is $$$\left[\begin{array}{cc}1 & - \frac{1}{a}\\0 & \frac{a + b}{a}\end{array}\right]$$$A.