Question
Simplify: $\text{cos}\theta\begin{bmatrix} \text{cos}\theta & \text{sin}\theta \\ \text{-sin}\theta & \text{cos}\theta \\ \end{bmatrix}+ \text{sin}\theta\begin{bmatrix} \text{sin}\theta & \text{-cos}\theta \\ \text{cos}\theta & \text{sin}\theta \\ \end{bmatrix}$.