#include <cmath> double acos( double arg );
The acos() function returns the arc cosine of arg, which will be in the range [0, pi]. arg should be between -1 and 1. If arg is outside this range, acos() returns NAN and raises a floating-point exception.
C++ also provides the following overloaded forms:
#include <cmath> float acos( float arg ); // same as acosf() in C99 long double acos( long double arg ); // same as acosl() in C99