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# C# Program to Calculate the Cosine(X) Without Using a Predefined Method

• Last Updated : 23 Dec, 2021

Cosine(x) is also known as Cos(x). It is a trigonometric function of an angle. The ratio of the length of the base to the length of the hypotenuse is known as the cosine of an angle in the right-angle triangle. In this article, we will learn to calculate the Cosine(X) without using a predefined method. So to do this task we use the following formula:

cos(x) = 1.0 – p /2 + q /24 – p * q /720 + q * q /40320 – p * q * q /3628800

Example:

```Input  : cos(45)
Output : 0.7071

Input  : cos(0)
Output : 1```

Code:

## C#

 `// C# program to calculate the Cosine(X)``// Without using a predefined method``using` `System;` `class` `GFG{` `// Function to calculate the Cosine(X)    ``static` `double` `Cos(``double` `theta)``{``    ``double` `R;``    ``double` `S;``    ``double` `ans;` `    ``R = Math.Pow(theta, 2);``    ``S = Math.Pow(R, 2);` `    ``// Substituting p,q in the below formula``    ``ans = 1.0 - R / 2 + S / 24 - R * S / 720 +``            ``S * S / 40320 - R * S * S / 3628800;` `    ``return` `ans;``}` `// Driver code``static` `void` `Main(``string``[] args)``{``    ``Console.WriteLine(``"Cos(0):"` `+ Cos(0));``    ``Console.WriteLine(``"Cos(3):"` `+ Cos(1));``    ``Console.WriteLine(``"Cos(8):"` `+ Cos(2));``}``}`

Output:

```Cos(0):1
Cos(3):0.540302303791887
Cos(8):-0.41615520282187```
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