acos, acosf, acosl - arc cosine function

Syntax	#include <math.h> double acos(double x); C11 float acosf(float x); long double acosl(long double x); (End)
Description	These functions are the inverse function of the corresponding cos()-functions. They return the principal value (i.e. corresponding angle in radians) of the arc cosine of a floating-point number x in the range [-1.0, +1.0].
Return val.	arc cosine(x) if successful. A floating-point number of type double in the range [0, pi] is returned. 0 if x does not lie in the range [-1.0, +1.0]. errno is set to indicate the error.
Errors	acos() will fail if: EDOM       The value of x is not in the range [-1.0, +1.0].
Note	To make sure you catch an error, you should set errno to 0 before calling acos(). If after execution errno != 0, an error has occurred.
See also	asin(), atan(), atan2(), cos(), sin(), tan(), math.h.