The matrices $\begin{bmatrix} \cos\theta &-\sin \theta \\ \sin \theta & cos \theta \end{bmatrix}$ and $\begin{bmatrix} a & 0 \\ 0 & b \end{bmatrix}$ commute under the multiplication
https://gateoverflow.in/2735/Gate-cse-1996-question-2-6
if a = b then commute property will satisfy
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