# Sine and Cosine of Supplementary Angles

## Theorem

### Sine of Supplementary Angle

$\sin \paren {\pi - \theta} = \sin \theta$

where $\sin$ denotes sine.

That is, the sine of an angle equals its supplement.

### Cosine of Supplementary Angle

$\map \cos {\pi - \theta} = -\cos \theta$

where $\cos$ denotes cosine.

That is, the cosine of an angle is the negative of its supplement.