用输入框输入一个整数n，
1、如果 n<3,则输出“数字过小”
2、如果3<=n<10，则输出1*2+2*3
+3
*4+…n*(n+1)的值
3、如果10<=N<100,则输出1+2+3+…+n
4、如果N>=100,则输出“数字过大”

Public Class Form1
Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
Dim n, tempNum As Integer
Dim s As String
'获取输入的内容
n = Int(Val(TextBox1.Text))
If n < 3 Then
s = "数字过小"
ElseIf 3 <= n And n < 10 Then
For i = 1 To n
tempNum = tempNum + i * (i + 1)
Next
s = Str(tempNum)
ElseIf 10 <= n And n < 100 Then
For i = 1 To n
tempNum = tempNum + i
Next
s = Str(tempNum)
ElseIf 100 <= n Then
s = "数字过大"
End If
Label1.Text = s
End Sub
End Class