1.4 acosh, acoshf—inverse hyperbolic cosine

Synopsis

#include <math.h>
double acosh(double x);
float acoshf(float x);

Description
acosh calculates the inverse hyperbolic cosine of x. acosh is defined as

 log(x + sqrt(x*x-1))

x must be a number greater than or equal to 1.

acoshf is identical, other than taking and returning floats.

Returns
acosh and acoshf return the calculated value. If x less than 1, the return value is NaN and errno is set to EDOM.

Portability
Neither acosh nor acoshf are ANSI C. They are not recommended for portable programs.