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# Technical Reference: Base Operating System and Extensions, Volume 1

## acosh, acoshf, or acoshl Subroutine

### Purpose

Computes the inverse hyperbolic cosine.

### Syntax

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

### Description

The acoshf, or acoshl subroutine
computes the inverse hyperbolic cosine of the x parameter.

The acosh subroutine returns the hyperbolic arc cosine
specified by the x parameter, in the range 1 to the
+HUGE_VAL value.

An application wishing to check for error situations should set errno to zero and call fetestexcept(FE_ALL_EXCEPT) before
calling these subroutines. Upon return, if the errno global
variable is nonzero or fetestexcept(FE_INVALID | FE_DIVBYZERO
| FE_OVERFLOW | FE_UNDERFLOW) is nonzero, an error has occurred.

### Parameters

x |
Specifies the value to be computed. |

### Return Values

Upon successful completion, the acoshf, or acoshl subroutine returns the inverse hyperbolic cosine of the given
argument.

For finite values of x < 1, a domain error occurs,
and a NaN is returned.

If x is NaN, a NaN is returned.

If x is +1, 0 is returned.

If x is +Inf, +Inf is returned.

If x is -Inf, a domain error occurs, and
a NaN is returned.

### Error Codes

The acosh subroutine returns NaNQ (not-a-number) and sets errno to EDOM if the x parameter is less than the value
of 1.

### Related Information

math.h in AIX 5L Version 5.2 Files Reference.

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