# Complex.Acos Method

.NET Framework 4.5

Returns the angle that is the arc cosine of the specified complex number.

Namespace:  System.Numerics
Assembly:  System.Numerics (in System.Numerics.dll)

## Syntax

```public static Complex Acos(
Complex value
)
```

#### Parameters

value
Type: System.Numerics.Complex

A complex number that represents a cosine.

#### Return Value

Type: System.Numerics.Complex
The angle, measured in radians, which is the arc cosine of value.

## Remarks

The Acos method for complex numbers corresponds to the Math.Acos method for real numbers.

The Acos method uses the following formula:

(-ImaginaryOne) * Log(value + ImaginaryOne * Sqrt(One - value * value)))

## Examples

The following example illustrates the Acos method. It shows that passing the value returned by the Acos method to the Cos method returns the original Complex value.

```using System;
using System.Numerics;

public class Example
{
public static void Main()
{
Complex[] values = { new Complex(.5, 2),
new Complex(.5, -2),
new Complex(-.5, 2),
new Complex(-.3, -.8) };
foreach (Complex value in values)
Console.WriteLine("Cos(ACos({0})) = {1}", value,
Complex.Cos(Complex.Acos(value)));
}
}
// The example displays the following output:
//       Cos(ACos((0.5, 2))) = (0.5, 2)
//       Cos(ACos((0.5, -2))) = (0.5, -2)
//       Cos(ACos((-0.5, 2))) = (-0.5, 2)
//       Cos(ACos((-0.3, -0.8))) = (-0.3, -0.8)
```

## Version Information

#### .NET Framework

Supported in: 4.5, 4

Supported in: 4

#### Portable Class Library

Supported in: Portable Class Library

#### .NET for Windows Store apps

Supported in: Windows 8

#### .NET for Windows Phone apps

Supported in: Windows Phone 8.1

## Platforms

Windows Phone 8.1, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.