# Arcvercosine
> Compute the [inverse versed cosine][inverse-versed-cosine].
The [inverse versed cosine][inverse-versed-cosine] is defined as
## Usage
```javascript
var avercos = require( '@stdlib/math/base/special/avercos' );
```
#### avercos( x )
Computes the [inverse versed cosine][inverse-versed-cosine].
```javascript
var v = avercos( 0.0 );
// returns 0.0
v = avercos( -3.141592653589793/2.0 );
// returns ~2.1783
v = avercos( -3.141592653589793/6.0 );
// returns ~1.0742
```
If `x < -2`, `x > 0`, or `x` is `NaN`, the function returns `NaN`.
```javascript
var v = avercos( 1.0 );
// returns NaN
v = avercos( -3.14 );
// returns NaN
v = avercos( NaN );
// returns NaN
```
## Examples
```javascript
var linspace = require( '@stdlib/array/linspace' );
var avercos = require( '@stdlib/math/base/special/avercos' );
var x = linspace( -2.0, 0.0, 100 );
var i;
for ( i = 0; i < x.length; i++ ) {
console.log( avercos( x[ i ] ) );
}
```
[inverse-versed-cosine]: https://en.wikipedia.org/wiki/Versine
