Question
If $A=\left[\begin{array}{ccc}4 & 3 & 2 \\ -1 & 2 & 0\end{array}\right], B=\left[\begin{array}{cc}1 & 2 \\ -1 & 0 \\ 1 & -2\end{array}\right]$, show that matrix $A B$ is non-singular.

Answer

$
\begin{aligned}
& AB =\left[\begin{array}{rrr}
4 & 3 & 2 \\
-1 & 2 & 0
\end{array}\right]\left[\begin{array}{rr}
1 & 2 \\
-1 & 0 \\
1 & -2
\end{array}\right] \\
& =\left[\begin{array}{rr}
4-3+2 & 8+0-4 \\
-1-2+0 & -2+0-0
\end{array}\right]=\left[\begin{array}{rr}
3 & 4 \\
-3 & -2
\end{array}\right] \\
& \therefore| AB |=\left|\begin{array}{rr}
3 & 4 \\
-3 & -2
\end{array}\right| \\
& \quad=-6-(-12)=6 \neq 0
\end{aligned}
$
Hence, $A B$ is a non-singular matrix.

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