帮我看看教科书上的一个例子

书上不可能是这样的,是你写错了,应该是这样的: (你看看有什么区别)

Dim a As Integer
Dim b As Integer
Dim c As Integer

Private Sub Command1_Click()
c = Val(Text1.Text)

If c = a + b Then
MsgBox "正确!", vbOKOnly, "提示"
a = Int(10 * Rnd): b = Int(10 * Rnd)
Label1.Caption = Str(a) & "+" & Str(b) & "="

Else
MsgBox "错误!", vbOKOnly + vbInformation, "提示"

End If
Text1.Text = ""
Text1.SetFocus
End Sub

Private Sub Form_Load()
Form1.Show

Randomize
a = Int(10 * Rnd): b = Int(10 * Rnd)

Label1.Caption = Str(a) & "+" & Str(b) & "="
Text1.Text = ""
Text1.SetFocus
Command1.Caption = "确认"

End Sub

a,b的定义在form_load中，所以a,b的赋值只有在这里有效，到了command1_click中a,b 的值就又成了空值。所以有误。将a,b定义为全局变量就可以了。改正后的程式如下：
Dim a As Integer
Dim b As Integer
Dim c As Inte