Matrix Inverse

cards/lin-alg
Inverse of $2\times 2$ matrix $A=\left[\begin{array}{cc}a& b\\ c& d\end{array}\right]$::$\frac{1}{ad-bc}\left[\begin{array}{cc}d& -b\\ -c& a\end{array}\right]$

Determinant of an invertible matrix::$\det A\ne 0$