Returns the angle whose cosine is the specified number.

## Syntax

#### Parameters

*d*-
A number representing a cosine, where
*d* must be greater than or equal to -1, but less than or equal to 1.

#### Returns

An angle, θ, measured in radians, such that 0 ≤ θ ≤ π

-or-

double.NaN if *d* < -1 or *d* > 1 or *d* equals double.NaN.

## Remarks

## Requirements

**Namespace: **System

**Assembly: **mscorlib (in mscorlib.dll)

**Assembly Versions: **1.0.5000.0, 2.0.0.0, 4.0.0.0