acos - arc cosine function
double acos(double x);
The acos() function calculates the arc cosine of x; that is
the value whose cosine is x. If x falls outside the range
-1 to 1, acos() fails and errno is set.
The acos() function returns the arc cosine in radians and
the value is mathematically defined to be between 0 and PI
EDOM x is out of range.
SVID 3, POSIX, BSD 4.3, ISO 9899
asin(3), atan(3), atan2(3),