C# has a built-in `Math` class that provides useful mathematical functions and operations. The `Math` class has the **`Acos()`** function, which is used to compute the *angle* whose *cosine* comes out to be a specified number.
This is also known as the **inverse cosine** of a specified number.

# Syntax
```
public static double Acos (double value);
```

# Parameters
- **`value`**: `value` is of the `Double` type and represents the input value, which is determined by finding `Acos()`. Its range must be: 

  - **-1 <= value <= 1**

# Return value
- **`Double`**: `Acos()` returns an angle Θ measured in radians, and of type `Double`, whose cosine is `value`. 
  - 0 <= Θ <= π (radians)

  It is true only for a valid range of `value`.
- **`NaN`**: `Acos()` returns `NaN` for an invalid range of `value`.

> Radians can be converted into degrees by multiplying radians by 180/Math.PI.

using System;
class Educative
{
    static void Main()
    {
        Double result = Math.Acos(0);
        System.Console.WriteLine("Acos(0) = "+ result + " radians");

        Double result1 = Math.Acos(1);
        System.Console.WriteLine("Acos(1) = "+ result1 + " radians");

        Double result2 = Math.Acos(-1);
        System.Console.WriteLine("Acos(-1) = "+ result2 + " radians");

        Double result3 = Math.Acos(2);
        System.Console.WriteLine("Acos(2) = "+ result3);
    }
}

What is Math.Acos() in C#?

Syntax

Parameters

Return value

Example