# Complex.Acos Method (Complex)

.NET Framework (current version)

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

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 4.0
Windows Phone
Available since 8.1