acos, acosf—arc cosine #include <math.h>
double acos(double x);
float acosf(float x);
Description
acos computes the inverse cosine (arc cosine) of the input value.
Arguments to acos must be in the range −1 to 1.
acosf is identical to acos, except that it performs
its calculations on floats.
Returns
acos and acosf return values in radians, in the range of 0 to pi.
If x is not between −1 and 1, the returned value is NaN
(not a number) the global variable errno is set to EDOM, and a
DOMAIN error message is sent as standard error output.
You can modify error handling for these functions using matherr.