# C# | Math.Acos() Method

Math.Acos() is an inbuilt Math class method which returns the angle whose cosine is given as a double value argument. If the argument is NaN, then the result will be NaN.

Syntax:

```public static double Acos(double num)
```

Parameter:

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.

Return Type: Returns an angle Θ, measured in radians and its type is System.Double. Here the angle is always measured in radians, such that 0 ≤ Θ ≤ π.

Note: If the value of the num is greater than 1 or less than -1 or equals to NaN, then this method always returns NaN as result. To convert the radians(return value) to degrees multiply it by 180 / Math.PI.

Examples:

```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
```

Program: To illustrate the Math.Acos() method

 `// 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)); ` `    ``} ` `} `

Output:

```NaN
NaN
1.20424285296577
90
1.5707963267949
NaN
NaN
NaN
```

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