Private Sub Command1_Click()

Dim i As Integer, j As Integer, a() As Double, n As Integer, x() As Double, sum As Double

ReDim Preserve a(1 To n, 1 To n + 1) As Double, x(1 To n) As Double

sum = 0
x(n) = a(n, n + 1) / a(n, n)
For i = (n - 1) To 1 Step -1
    For j = (i + 1) To n Step 1
        sum = sum + a(i, j) * x(j)
    Next j
    x(i) = (a(i, n + 1) - sum) / a(i, i)
Next i
End Sub
a()和x()已经定义成公共变量了
这段程序怎么有问题啊！调试的时候讲是小标超界，这到底是怎么回事啊？

Private Sub Command1_Click()
Dim i As Integer, j As Integer, a() As Double, n As Integer, x() As Double, sum As Double

ReDim Preserve a(1 To n, 1 To n + 1) As Double, x(1 To n) As Double
重定义数组大小
这时，N没有值，所以=0 这条命令就等价于
ReDim Preserve a(1 To 0, 1 To 0 + 1) As Double, x(1 To 0) As Double
不报错才有鬼呢。
sum = 0

x(n) = a(n, n + 1) / a(n, n)
因为数据没有初始化过，所以全是0，
所以这名，相当于 x(n)=0/0 ,继续报错。

For i = (n - 1) To 1 Step -1
   因为 n=0，所以此循环是 -1 to 1
    For j = (i + 1) To n Step 1
      此循环是 0 t 0
        sum = sum + a(i, j) * x(j)
        此结果是 0+0*0
    Next j
    x(i) = (a(i, n + 1) - sum) / a(i, i)
   最后的结果是 0/0 继续报错。
Next i
End Sub

[[it] 本帖最后由 风吹过b 于 2008-11-4 21:50 编辑 [/it]]

如果你定义了公共变量的话，那么在过程里，就不要再重新定义，如果重新定义了，就是模块变量，这时，对此变量的引用，都是引用这个模块变量，不会引用于公共变量去。