Synopsis
double acos(double x);
Description

acos returns the principal value, in radians, of the inverse circular cosine of x. The principal value lies in the interval [0, PI] radians.

If |x| > 1, errno is set to EDOM and acos returns HUGE_VAL.

If x is NaN, acos returns x. If |x| > 1, acos returns NaN.