## ACOS

The `ACOS` function in ES|QL returns the arccosine of a number as an angle, expressed in radians. The number should be between -1 and 1. If the input is null, the function will return null.

### Syntax

`ACOS(number)`

#### Parameters

`number`: A number between -1 and 1. If null, the function returns null.

### Examples

Here are a couple of examples of how to use the `ACOS` function in ES|QL queries:

```esql
ROW a=.9
| EVAL acos = ACOS(a)
```

In this example, the `ACOS` function is used to calculate the arccosine of 0.9.

```esql
ROW a=-.5
| EVAL acos = ACOS(a)
```

In this example, the `ACOS` function is used to calculate the arccosine of -0.5.