请大家帮忙看看我编的龙格库塔法为什么不对，结果老不正确

Public Class Form1
    Public K1 As Double, K2 As Double, K3 As Double, K4 As Double
    Public h As Double, n As Integer, a As Double, b As Double
    Public xk(1000) As Double, yk(1000) As Double, x0 As Double, y0 As Double
    Public result As Double
    Public Function f(ByVal x As Double, ByVal y As Double) As Double
        Dim res As Double
        res = y - 2.0 * x / y
        Return res
    End Function
    Public Function COMpute() As Double
        Dim i As Integer
        xk(0) = x0 : yk(0) = y0
        For i = 0 To n
            xk(i + 1) = xk(i) + h
            K1 = h * f(xk(i), yk(i))
            K2 = h * f(xk(i) + 0.5 * h, yk(i) + 0.5 * K1)
            K3 = h * f(xk(i) + 0.5 * h, yk(i) + 0.5 * K2)
            K4 = h * f(xk(i) + h, yk(i) + K3)
            yk(i + 1) = yk(i) + (K1 + 2 * K2 + 2 * K3 + K4) / 6
        Next
        Return yk(i)
    End Function
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCompute.Click
        a = Val(txta.Text) : b = Val(txtb.Text) : h = Val(txth.Text)
        n = (b - a) / h
        x0 = Val(txtx0.Text) : y0 = Val(txty0.Text)
        result = COMpute()
        txtResult.Text = result
    End Sub
End Class
这个是代码，图形界面是看附件，版主，帮帮忙啊，您最厉害了