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## 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 = [itex]\alpha[/itex][itex]\frac{R4}{R4 + Ri}[/itex]

## The Attempt at a Solution

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

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