如何在循环中在Richtextbox中显示输出

Question

我正在尝试获取一个RichTextBox，以在单击按钮时显示循环的输出。我该怎么做呢？这是有问题的代码部分

Private Sub calculate_btn_Click(sender As Object, e As EventArgs) Handles calculate_btn.Click
    interest = principal = ((1 + roi / compoundFreq) ^ (compoundFreq * numbryears) - 1)
    numbryears = endyear - startyear

    Dim output_txt As Integer
    For thisyear = startyear To endyear
        output_txt += thisyear
    Next thisyear

End Sub