Program:

The source code to calculate the Cosine(X) without using a predefined method is given below. The given program is compiled and executed successfully on Microsoft Visual Studio.

//C# program to calculate the Cosine(X) 
//without using a predefined method.

using System;

class Demo
{
    static double CalculateCosX(double X)
    {
        double P = 0.0F;
        double Q = 0.0F;
        double Result = 0.0F;

        P = Math.Pow(X, 2);
        Q = Math.Pow(P, 2);

        Result = 1.0 - P / 2 + Q / 24 - P * Q / 720 + Q * Q / 40320 - P * Q * Q / 3628800;

        return Result;
    }
    static void Main(string[] args)
    {
        double val = 0.0F;

        while (val <= 5)
        {
            Console.WriteLine("Cosine({0}) => {1}", val, CalculateCosX(val));
            val++;
        }
    }
}

Output:

Cosine(0) => 1
Cosine(1) => 0.540302303791887
Cosine(2) => -0.41615520282187
Cosine(3) => -0.991049107142857
Cosine(4) => -0.6857848324515
Cosine(5) => -0.162746638007054
Press any key to continue . . .

Explanation:

here, we created a class Demo that contains two static methods CalculateCosX() and Main() method.

In the CalculateCosX() method, we calculate cosine using the below formula.
Cos(x) = 1.0- p /2+ q /24- p * q /720+ q * q /40320- p * q * q /3628800;

The Main() is the entry point of the program, here we called the CalculateCosX() method for values 0 to 5 using the "while" loop and print the results on the console screen.