# acos
> Compute the [arccosine][arccosine] of a number.
## Usage
```javascript
var acos = require( '@stdlib/math/base/special/acos' );
```
#### acos( x )
Computes the [arccosine][arccosine] of a `number` (in radians).
```javascript
var v = acos( 1.0 );
// returns 0.0
v = acos( 0.707 ); // ~pi/4
// returns ~0.7855
v = acos( 0.866 ); // ~pi/6
// returns ~0.5236
v = acos( NaN );
// returns NaN
```
The domain of `x` is restricted to `[-1,1]`. If `|x| > 1`, the function returns `NaN`.
```javascript
var v = acos( -3.14 );
// returns NaN
```
## Examples
```javascript
var linspace = require( '@stdlib/array/linspace' );
var acos = require( '@stdlib/math/base/special/acos' );
var x = linspace( -1.0, 1.0, 100 );
var i;
for ( i = 0; i < x.length; i++ ) {
console.log( acos( x[ i ] ) );
}
```
[arccosine]: https://en.wikipedia.org/wiki/Inverse_trigonometric_functions