\[
   \cos{(\theta_2-\theta_1)}
   =
   \begin{pmatrix}\cos{\theta_1}\\\sin{\theta_1}\end{pmatrix}
   \cdot
   \begin{pmatrix}\cos{\theta_2}\\\sin{\theta_2}\end{pmatrix}
   =
   \cos{\theta_1}\cos{\theta_2}
   +
   \sin{\theta_1}\sin{\theta_2}
\]