Single Transistor Overall Current Gain

[hisotaso]

Homework Statement

THe problem statement and given solutions are in this thread:

https://www.physicsforums.com/showthread.php?t=694054

I do not understand the equation for current gain:

Homework Equations

Ai = $\alpha$$\frac{R4}{R4 + Ri}$

The Attempt at a Solution

I have pages of work on this, but basically what I know is $\frac{Io}{Ie}$ = $\alpha$

then $\frac{Ie}{Ii}$ must equal $\frac{R4}{R4 + Ri}$, I don't see that.

 

[NascentOxygen]

Why have you started a second thread for your own question?

 

[hisotaso]

I started a second thread because I thought I had figured it out. I stated so in that thread, and thus expected no one would reply. I asked another question about this same circuit in a third thread.