how can i find value of cos without using math library of c#
.......plz send me code example or formula by which it ia possible i thank full to u

Did you search the web? Or have you absolutely no clue as how to tackle your problem?

Because cos(x) can be writen as an infinite series. See picture included. You could do something like this :

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.
        }
    }
}

A Taylor series isn't really a good way to compute this function unless you're looking for asymptotic accuracy around a particular point, rather than general accuracy along the whole thing. Also, it's a good idea to start by folding the number down to the interval [0,pi]. But suppose we did want to go with a Taylor series because that's all we knew of.

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))));
        }
    }
}

See pages 115 and 116 here for a comparison of a Taylor approximation with one that strives for a different metric of accuracy.