Cosine of Angle plus Straight Angle

Theorem

 * $\cos \paren {x + \pi} = -\cos x$

Also see

 * Sine of Angle plus Straight Angle
 * Tangent of Angle plus Straight Angle
 * Cotangent of Angle plus Straight Angle
 * Secant of Angle plus Straight Angle
 * Cosecant of Angle plus Straight Angle