acos

#include <cmath>float acos(float arg);double acos(double arg);
long double acos(long double arg);

The acos( ) function returns the arc cosine of arg. The argument to acos( ) must be in the range –1 to 1; otherwise, a domain error will occur.

Related functions are asin( ), atan( ), atan2( ), sin( ), cos( ), tan( ), sinh( ), cosh( ), and tanh( ).