Simplify: `costheta[costhetasintheta-sinthetacostheta]+sintheta[sintheta-costhetacosthetasintheta]`

Explanation

`Cos theta**[[cos theta,sin theta],[-sin theta,costheta]] +sin theta [[sin theta, - cos theta],[cos theta,sin theta]]`


$$=\left[\begin{array}{cc}\cos ^{2} \theta & \sin \cdot \cos \theta \\ -\cos \cdot \sin \theta & \cos ^{2} \theta\end{array}\right]+\left[\begin{array}{cc}\sin ^{2} \theta & -\sin \theta \cos \theta \\ \sin \theta \cdot \cos \theta & \sin ^{2} \theta\end{array}\right]$$
$$=\left[\begin{array}{ll}1 & 0 \\ 0 & 1\end{array}\right]$$
$$=\mathrm{I}_{2 \times 2}$$