### NAME

acosh, acoshf, acoshl - inverse hyperbolic cosine function

### SYNOPSIS

**#include <math.h>**

**double acosh(double ***x***);**
**float acoshf(float ***x***);**
**long double acoshl(long double ***x***);**

Link with *-lm*.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

**acosh**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE; or
*cc -std=c99* **acoshf**(), **acoshl**(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or
*cc -std=c99*

### DESCRIPTION

The
**acosh**() function calculates the inverse hyperbolic cosine of
*x*; that is the value whose hyperbolic cosine is *x*.

### RETURN VALUE

On success, these functions return the inverse hyperbolic cosine of
*x*.

If
*x* is a NaN, a NaN is returned.

If
*x* is +1, +0 is returned.

If
*x* is positive infinity, positive infinity is returned.

If
*x* is less than 1,
a domain error occurs,
and the functions return a NaN.

### ERRORS

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

Domain error: x is less than 1
| |

errno is set to
EDOM. An invalid floating-point exception
(FE_INVALID) is raised.
| |

### CONFORMING TO

C99, POSIX.1-2001.
The variant returning
*double* also conforms to
SVr4, 4.3BSD, C89.

### SEE ALSO

asinh(3), atanh(3), cacosh(3), cosh(3), sinh(3), tanh(3)

### COLOPHON

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