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




namespace ConsoleApplication2
{

    class Program

{


        public static double fun(double x, double y, double n,double T)
        {
            double tau = (T) / (n - 1);
            return (Cos(0.8 * tau * y) - x);
        }

        static void Main(string[] args)

    {


        //задача о вынужденных колебаниях

        //метод эйлера

        int n = 1000;

        double w0 = 1;

        double lam = 0.8;

        double t0 = 0;

        double T = 10*PI;

        double x0 = 0; double v0 = 0;//начальные условия

        double[] vn = new double[n + 1];

        vn[0] = v0;

        double[] xn = new double[n + 1];

        xn[0] = x0;

        double tau = (T - t0) / (n-1);

        System.IO.StreamWriter textFile1 =

        new System.IO.StreamWriter(@"E:\8_euler.txt");
            double t=0;
        for (int i = 1; i <= n; i++)

        {
                t = i;
            vn[i] = vn[i - 1] + tau * (- w0 * w0 * xn[i - 1]+Cos(lam*tau*t));

            xn[i] = xn[i - 1] + tau * vn[i - 1];

            Console.Write(xn[i] + " ");

            textFile1.WriteLine(xn[i]);

        }

        textFile1.Close();

        //метод эйлера-кромера

        System.IO.StreamWriter textFile2 =

        new System.IO.StreamWriter(@"E:\8_eulerkromer.txt");

        for (int i = 1; i <= n; i++)

        {

                vn[i] = vn[i - 1] + tau * (-w0 * w0 * xn[i - 1] + Cos(lam * tau * t));

                xn[i] = xn[i - 1] + tau * vn[i];

                Console.Write(xn[i] + " ");

            textFile2.WriteLine(xn[i]);

        }

        textFile2.Close();




        //метод рунге кутта

        System.IO.StreamWriter textFile3 =

        new System.IO.StreamWriter(@"E:\8_rungekutt.txt");

        for (int i = 1; i <= n; i++)

        {
                t = i;
                double k0 = fun(xn[i - 1],t,n, T) ;

            double k1 = fun(xn[i - 1]+tau/2, t, n, T);

            double k2 = fun(xn[i - 1]+tau/2, t, n, T);

            double k3 = fun(xn[i - 1] + tau, t, n, T);

            vn[i] = vn[i - 1] + tau / 6 * (k0 + 2 * k1 + 2 * k2 + k3);

            xn[i] = xn[i - 1] + tau * vn[i];

            Console.Write(xn[i] + " ");

            textFile3.WriteLine(xn[i]);

        }

        textFile3.Close();

    }

}

}