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### 1.3 `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`.

You can change the error-handling behavior with the non-ANSI `matherr` function.

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