Cos[z] gives the cosine of z.
Mathematical function (see Section A.3.10).
The argument of Cos is assumed to be in radians. (Multiply by Degree to convert from degrees.)
Cos is automatically evaluated when its argument is a simple rational multiple of ; for more complicated rational multiples, FunctionExpand can sometimes be used.
See The Mathematica Book: Section 1.1.3 and Section 3.2.6.
See also: ArcCos, Sec, TrigToExp, TrigExpand.