##### 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).