Half-Angle Identities

Identity
\[ \sin^2 x = \frac{1}{2}(1 - \cos 2x) \]
\[ \cos^2 x = \frac{1}{2}(1 + \cos 2x) \]
\[ \tan^2 x = \frac{1 - \cos 2x}{1 + \cos 2x} \]