Cosine of Angle plus Full Angle

Theorem

 * $\cos \paren {x + 2 \pi} = \cos x$

Also see

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