Double - Acos

Returns the angle whose cosine is the specified number.

Try it

public static void Main()
{
	double[] doubles = { 0, 0.333, 0.667, 1 };
	
  	foreach (double value in doubles)
	{
		// C# Extension Method: Double - Acos
    	Console.WriteLine("Acos({0}) = {1}", value, value.Acos());
	}
}

View Source 
using System;

public static partial class Extensions
{
    /// <summary>
    ///     Returns the angle whose cosine is the specified number.
    /// </summary>
    /// <param name="d">
    ///     A number representing a cosine, where  must be greater than or equal to -1, but less than or
    ///     equal to 1.
    /// </param>
    /// <returns>An angle, ?, measured in radians, such that 0 ????-or-  if  &lt; -1 or  &gt; 1 or  equals .</returns>
    public static Double Acos(this Double d)
    {
        return Math.Acos(d);
    }
}