# llAcos

*LSL function*

```lsl
float float llAcos(float val);
```

- `float val`: A number in the range [-1.0, 1.0].

- Returns: `float`
- Energy: 10

The returned value is in the range \[0.0, [PI](/constants/PI/)\], or NaN signifying a complex result when the parameter is out of range.

Returns a [float](https://wiki.secondlife.com/wiki/float) that is the arccosine in radians of **val**

```lsl title="How to use" frame="terminal"
float result = llAcos(0.0);
```

## Caveats

- Triggers a [Math Error](https://wiki.secondlife.com/wiki/Category:LSL_Error/Math) for complex results if not compiled in Mono.

## Examples

<details open>
<summary>Example 1</summary>

```lsl
//  This example exercises the full valid range of argument from -1.0 to +1.0

default
{
    touch_start(integer num_detected)
    {
        float random = llFrand(2.0) - 1.0;

        llOwnerSay("The arccosine of " + (string)random + " is " + (string)llAcos(random));
    }
}
```

</details>

<details>
<summary>Example 2</summary>

```lsl
// An example for the Law of Cosines to calculate any angle of a triangle from 3 known side lengths.
default
{
    touch_start(integer num_detected)
    {
        vector t = <llFrand(9.0)+1.0, llFrand(9.0)+1.0, llFrand(9.0)+1.0>; // The 3 values of this vector are used to set the lengths of 3 sides of a triangle.

        float anglexy = llAcos((t.x*t.x+t.y*t.y-(t.z*t.z)) / ( 2*t.x*t.y)); // Law Of Cosines

        llOwnerSay("Using the Law Of Cosines, the angle of a triangle between side a=" + (string)t.x + "and side b=" + (string)t.y + " is = " + (string)anglexy + " radians=" + (string)(anglexy * RAD_TO_DEG) + "° and that angle is not on side c=" + (string)t.z);
    }
}
```

</details>

## See also

### Functions

- [llSin](/functions/llSin/) — [llAsin](/functions/llAsin/) — sine & inverse Sine
- [llCos](/functions/llCos/) — cosine
- [llTan](/functions/llTan/) — [llAtan2](/functions/llAtan2/) — tangent & inverse tangent

### Articles

- [Inverse trigonometric function](https://en.wikipedia.org/wiki/Inverse_trigonometric_function)

---

*Source: [LlAcos](https://wiki.secondlife.com/wiki/LlAcos) on the Second Life Wiki. Content from the Second Life Wiki article LlAcos (revision 1218950, 2026-07-15), CC BY-SA 3.0.*

---

From lsl.dev: https://lsl.dev/functions/llAcos/