Cosine of Supplementary Angle

Theorem

 * $\cos \paren {\pi - \theta} = - \cos \theta$

where $\cos$ denotes cosine.

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

Also see

 * Sine of Supplementary Angle
 * Tangent of Supplementary Angle
 * Cotangent of Supplementary Angle
 * Secant of Supplementary Angle
 * Cosecant of Supplementary Angle