\begin{eqnarray*}
\sin^{2}\theta & = & \frac{1-\cos2\theta}{2}\\
\cos^{2}\theta & = & \frac{1+\cos2\theta}{2}\\
\tan^{2}\theta & = & \frac{1-\cos\theta}{1+\cos\theta}
\end{eqnarray*}