c# Wahadło matematyczne

Potrzebuję przerobić to wahadło które wykonane jest metodą Runge-Kutta 4 rzedu, na wykonane metoda Runge-Kutta 2 rzedu,
w tym uzyta zostala gotowa biblioteka, a ja potrzebuję metode rozwiązująca równanie różniczkowe RK2, niestety z fizyki jestem nogą, proszę o pomoc, sugestie, podpowiedzi

 using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;
namespace Swing
{

public partial class Pendulum : Window
{
    private double PendulumMass = 1;
    private double PendulumLength = 1;
    private double DampingCoefficient = 0.5;
    private double Theta0 = 45;
    private double Alpha0 = 0;
    double[] xx = new double[2];
    double time = 0;
    double dt = 0.03;
    Polyline pl = new Polyline();
    double xMin = 0;
    Double yMin = -100;
    double xMax = 50;
    double yMax = 100;
    public Pendulum()
{
    InitializeComponent();
}


private void btnStart_Click(object sender, RoutedEventArgs e)
{
    PendulumMass = Double.Parse(tbMass.Text);
    PendulumLength = Double.Parse(tbLength.Text);
    DampingCoefficient = Double.Parse(tbDamping.Text);
    Theta0 = Double.Parse(tbTheta0.Text);
    Theta0 = Math.PI * Theta0 / 180;
    Alpha0 = Double.Parse(tbAlpha0.Text);
    Alpha0 = Math.PI * Alpha0 / 180;
    tbDisplay.Text = "Starting...";

    if (canvasRight.Children.Count > 4)
    canvasRight.Children.Remove(pl);

    pl = new Polyline();
    pl.Stroke = Brushes.Black;
    canvasRight.Children.Add(pl);
    time = 0;
    xx = new double[2] { Theta0, Alpha0 };
    CompositionTarget.Rendering += StartAnimation;
}

private void StartAnimation(object sender, EventArgs e)
{
// Invoke ODE solver:
    ODESolver.Function[] f = new ODESolver.Function[2] { f1, f2 };
    double[] result = ODESolver.RungeKutta4(f, xx, time, dt);

// Display moving pendulum on screen:
    Point pt = new Point(140 + 130 * Math.Sin(result[0]),20 + 130 * Math.Cos(result[0]));
    ball.Center = pt;
    line1.X2 = pt.X;
    line1.Y2 = pt.Y;

// Display theta - time curve on canvasRight:
    if (time < xMax)
    pl.Points.Add(new Point(XNormalize(time) + 10,
    YNormalize(180 * result[0] / Math.PI)));
// Reset the initial values for next calculation:
    xx = result;
    time += dt;
    if (time > 0 && Math.Abs(result[0]) < 0.01 &&
    Math.Abs(result[1]) < 0.001)

{
    tbDisplay.Text = "Stopped";
    CompositionTarget.Rendering -= StartAnimation;
}

}

private void btnReset_Click(object sender, RoutedEventArgs e)
{
    PendulumInitialize();
    tbDisplay.Text = "Stopped";
    if (canvasRight.Children.Count > 4)
    canvasRight.Children.Remove(pl);
    CompositionTarget.Rendering -= StartAnimation;
}

private void PendulumInitialize()
{
    tbMass.Text = "1";
    tbLength.Text = "1";
    tbDamping.Text = "0.1";
    tbTheta0.Text = "45";
    tbAlpha0.Text = "0";
    line1.X2 = 140;
    line1.Y2 = 150;
    ball.Center = new Point(140, 150);
}

private void btnStop_Click(object sender, RoutedEventArgs e)
{
    line1.X2 = 140;
    line1.Y2 = 150;
    ball.Center = new Point(140, 150);
    tbDisplay.Text = "Stopped";
    CompositionTarget.Rendering -= StartAnimation;
}

private double f1(double[]xx, double t)
{
    return xx[1];
}
private double f2(double[] xx, double t)
{
    double m = PendulumMass;
    double L = PendulumLength;
    double g = 9.81;
    double b = DampingCoefficient;
    return -g * Math.Sin(xx[0]) / L - b * xx[1] / m;
}
private double XNormalize(double x)
{
    double result = (x - xMin) * canvasRight.Width / (xMax - xMin);
    return result;
}
private double YNormalize(double y)
{
    double result = canvasRight.Height - (y - yMin) * canvasRight.Height / (yMax - yMin);
    return result;
}
}
}