If `\alpha - \beta = (2n+1)\frac{\pi}{2}` where `n in \mathbb{Z}` then show that product of $ \begin{pmatrix} \cos^2\alpha & \cos\alpha\sin\alpha \\ \cos\alpha\sin\alpha & \sin^2\alpha \end{pmatrix} $  and  $ \begin{pmatrix} \cos^2\beta& \cos\beta\sin\beta \\ \cos\beta\sin\beta & \sin^2\beta \end{pmatrix} $ is a zero matrix.