using System;
 
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            for (double d = 0; d < 3.0; d += 0.4)
            {
                Console.WriteLine("The cosine of {0} = {1}", d, Math.Cos(d));
                Console.WriteLine("Calculated cosine of {0} = {1}", d, cos(d));
                Console.WriteLine();
            }
            Console.ReadKey(); //keep console on screen until key press
        }
 
        static double cos(double x) //calculate cosine
        {
            double p = x * x; //reduce amount of multiplications
            double q = p * p;
            return 1.0 - p / 2 + q / 24 - p * q / 720 + q * q / 40320 - p * q * q / 3628800;
            // add more terms for more accuracy
           // 2, 24,720 etc. are the factorials of 2,4,6 etc.
        }
    }
}

        static double cos(double x) //calculate cosine
        {
            // move x to value in [0, pi] with equal answer
            x = Math.Abs((x + Math.PI) % (2 * Math.PI) - Math.PI);
            const double tf = 1.0 / 24.0, vtz = -1.0 / 720.0, fzhtz = 1.0 / 40320.0, fukit = -1.0 / 3628800.0;
            double p = x * x;
            // use Horner's method instead, just because.
            return 1 + p * (-0.5 + p * (tf + p * (vtz + p * (fzhtz + p * fukit))));
        }
    }
}