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？