## NAME

acos - arc cosine function

## SYNOPSIS

**#include** **<math.h>**
**double** **acos(double** **x);**

## DESCRIPTION

The **acos()** function calculates the arc cosine of **x**; that is
the value whose cosine is **x**. If **x** falls outside the range
-1 to 1, **acos()** fails and *errno* is set.

## RETURN VALUE

The **acos()** function returns the arc cosine in radians and
the value is mathematically defined to be between 0 and PI
(inclusive).

## ERRORS

**EDOM** **x** is out of range.

## CONFORMING TO

SVID 3, POSIX, BSD 4.3, ISO 9899

## SEE ALSO

**asin**(3), **atan**(3), **atan2**(3),