double acosh(double x);
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.
If x < 1, errno is set to EDOM and acosh returns HUGE_VAL.
If x < 1, acosh returns NaN.
If x is NaN, acosh returns NaN.