\begin{equation}
   \underline {\underline {\mathbf{M}}}(\theta)=
   \begin{pmatrix}
      \cos{\theta} & -\sin{\theta}\\
      \sin{\theta} & \cos{\theta}
   \end{pmatrix}
\end{equation}

\begin{equation}
   \left| \begin{array}{rr}
      \cos{\theta} & -\sin{\theta}\\
      \sin{\theta} & \cos{\theta}
      \end{array}
   \right|
   =
   \cos^2{\theta}+\sin^2{\theta}
\end{equation}

