Simplify `costheta[[costheta,sintheta],[-sintheta,costheta]]+sintheta[[sintheta,-costheta],[costheta,sintheta]]`

Explanation

To simplify the expression \( \cos \theta \begin{pmatrix} \cos \theta & \sin \theta \\ -\sin \theta & \cos \theta \end{pmatrix} + \sin \theta \begin{pmatrix} \sin \theta & -\cos \theta \\ \cos \theta & \sin \theta \end{pmatrix} \), we will follow these steps:

### Step 1: Multiply the first matrix by \( \cos \theta \)

We start with the first term:
\[ \cos \theta \begin{pmatrix} \cos \theta & \sin \theta \\ -\sin \theta & \cos \theta \end{pmatrix} \]
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