求解一元方程组 跪求 急！急！！！
方程一：Y= aX/[1+(a-1)X]
方程二：Y(n+1)=RXn/(1+R)+Xd/(1+R)
已知：Xd，a，R，Xq为常数。
现：用Y1=Xd，代入方程一求的X1， 再代入方程二求出Y2
再代入方程一求X2，......直到 Xn <=Xq
求的n的值~~
应该蛮简单的，但刚学vb，不怎么会做````

[解决办法]
Private Sub Command1_Click()
Dim a As Single, R As Single, Xd As Single, Xq As Single
Dim n As Integer, X() As Single, Y() As Single
Xd = 0.01: a = 3: R = 4: Xq = 0.42
n = 1
Do
ReDim X(n + 1)
ReDim Y(n + 1)
Y(n) = Xd
X(n) = Y(n) / (a - (a - 1) * Y(n))
Y(n + 1) = R * X(n) / (1 + R) + Xd / (1 + R)
Y(n) = Y(n + 1)
n = n + 1
Loop Until X(n) <= Xq
Print "n= "; n

End Sub