S = 1 + x^2/2! + x^4/4! +...+ x^2n/2n!

giai-bai-tap.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace GiaiBaiTap
{
    class Program
    {
        static void Main(string[] args)
        {
            int x = 3;
            int n = 4; 
            Console.WriteLine(TinhS(x,n));
            Console.ReadLine(); 
        }

        static double TinhS(int x, int n)
        {
            double tmp = 1;
            for (int i = 1; i <= n; i++)
            {
                return tmp += Math.Pow(x, 2 * i) / GiaiThua(2 * i);
            }
            return tmp; 
        }

        static long GiaiThua(int x)
        {
            long tmp = 1;
            for (int i = 1; i <= x; i++)
            {
                tmp *= i; 
            }
            return tmp; 
        }
    }
}