#include <math.h>
float acoshf(float x);
acoshf returns the non-negative inverse hyperbolic cosine of x.
If x< 1, errno is set to EDOM and acoshf returns HUGE_VALF.
If x < 1, acoshf returns NaN with signal.
If x is NaN, acoshf returns NaN without signal.
acoshf conforms to ISO/IEC 9899:1999 (C99).