Synopsis
#include <math.h>
double acosh(double x);
Description
acosh returns the non-negative inverse hyperbolic cosine of x.
acosh(x) is defined as log(x + sqrt(x^2-1)), assuming completely accurate computation.
Fast math library behavior
If x< 1, errno is set to EDOM and acosh returns HUGE_VAL.
IEC 60559 math library behavior
If x < 1, acosh returns NaN with signal.
If x is NaN, acosh returns NaN without signal .
Portability
acosh conforms to ISO/IEC 9899:1999 (C99).