---
title: math.acos
summary: null
url: >-
  https://www.fastly.com/documentation/reference/vcl/functions/math-trig/math-acos
---

```
FLOAT math.acos(FLOAT x)
```

**Available in:** all subroutines

Computes the principal value of the arc cosine of its argument `x`.

## Parameters

`x` - Floating point value. The value of `x` should be in the range -1 to 1
inclusive.

## Return Value

Upon successful completion, this function returns the arc cosine of `x` in the
range 0 to `math.PI` radians inclusive.

If `x` is `math.NAN`, a NaN will be returned.

If `x` is +1, +0 will be returned.

If `x` is `math.POS_INFINITY` or
`math.NEG_INFINITY`, a domain error occurs
and a NaN will be returned.

For finite values of x not in the range -1 to 1 inclusive, a domain error occurs
and a NaN will be returned.

## Errors

If the `x` argument is finite and is not in the range -1 to 1 inclusive,
or is `math.POS_INFINITY` or
`math.NEG_INFINITY`,
then `fastly.error` will be set to `EDOM`.

## Example

```vcl
declare local var.fo FLOAT;

set var.fo = math.cos(1.1); // Returns math.NAN

if (fastly.error) {
  set resp.http.acos-error = fastly.error; // Returns "EDOM"
}
```