Taylor's polynomial of cos(2x) -e^-x

[vonbv25]

n = Integer.Parse(iVal.Text)
        x = Double.Parse(xVal.Text)

        '  fpoly = taylors(iter, x)
        factorial = 1
        poly = 0
        Dim a As Integer
        For a = 1 To n
       
            factorial = factorial * a
            If factorial Mod 2 = 0 Then
                evenfac = factorial
            Else
                oddfac = factorial
            End If
            poly = poly + (Math.Pow(x, ((2 * a) - 1)) / ((oddfac)) + ((Math.Pow(2, (2 * a)) - Math.Pow(1, a)) - 1) * (Math.Pow(x, (2 * a)) / (evenfac)))
        Next a


        RichTextBox1.Text = poly.ToString