Math.Acos()是内置的Math类方法，该方法返回以余弦值作为双值参数给出的角度。如果参数为NaN ，则结果将为NaN 。
句法：
public static double Acos(double num)
范围：
num: It is the number that represents cosine and type of this parameter is System.Double. It must be greater than or equal to -1, but less than or equal to 1.
返回类型：返回以弧度为单位的角度Θ，其类型为System.Double 。在此，角度始终以弧度为单位，以使0≤Θ≤π。
注意：如果num的值大于1或小于-1或等于NaN ，则此方法始终返回NaN作为结果。要将弧度(返回值)转换为度数，请乘以180 / Math.PI。
例子：
Input : Math.Acos(2)
Output : NaN
Input : Math.Acos(0.3584)
Output : 1.20424285296577
Input : Math.Acos(0.0)
Output : 1.5707963267949
Input : Math.Acos(-0.0)
Output : 1.5707963267949
Input : Math.Acos(Double.PositiveInfinity)
Output : NaN
Input : Math.Acos(Double.NegativeInfinity)
Output : NaN
程序：演示Math.Acos()方法
// C# program to demonstrate working
// of Math.Acos() method
using System;
class Geeks {
// Main Method
public static void Main(String[] args)
{
double a = Math.PI;
// using Math.Acos() method
Console.WriteLine(Math.Acos(a));
// argument is greater than 1
Console.WriteLine(Math.Acos(2));
Console.WriteLine(Math.Acos(0.3584));
double d = 0.0;
double e = -0.0;
double posi = Double.PositiveInfinity;
double nega = Double.NegativeInfinity;
double nan = Double.NaN;
// Input positive zero
// Output 1.5707963267949
double res = Math.Acos(d);
// converting to degree
// i.e output will be 90 degree
double rest = res * (180 / Math.PI);
Console.WriteLine(rest);
// Input negative zero
// Output 1.5707963267949
Console.WriteLine(Math.Acos(e));
// input PositiveInfinity
// Output NaN
Console.WriteLine(Math.Acos(posi));
// input NegativeInfinity
// Output NaN
Console.WriteLine(Math.Acos(nega));
// input NaN
// Output NaN
Console.WriteLine(Math.Acos(nan));
}
}
输出：
NaN
NaN
1.20424285296577
90
1.5707963267949
NaN
NaN
NaN
参考： https://msdn.microsoft.com/en-us/library/system.math.Acos