Half Angle Formula for Tangent/Corollary 2

Theorem

 * $\tan \dfrac \theta 2 = \dfrac {1 - \cos \theta} {\sin \theta}$

where $\tan$ denotes tangent, $\sin$ denotes sine and $\cos$ denotes cosine.

When $\theta = \paren {2 k + 1} \pi$, $\tan \dfrac \theta 2$ is undefined.

Also see

 * Half Angle Formula for Sine
 * Half Angle Formula for Cosine