Compute … and … and also … and …:

Compute $AB$ and $BA$ and also $BC$ and $CB$:
$A=\left[\begin{array}{cc}1& 4\\ 2& -1\end{array}\right]B=\left[\begin{array}{cc}3& 1\\ 1& 1\end{array}\right]C=\left[\begin{array}{cc}1& 1\\ 0& 2\end{array}\right]$.
Verify the associative law: $AB$ times $C$ equals $A$ times $BC$.

Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §11.4 Matrices and Linear Equations, #29