- #1

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- 9

## Homework Statement

## Homework Equations

## The Attempt at a Solution

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Hi,

I am trying to understand the 2nd equality .

I thought perhaps it is an expansion of ##(1-\frac{z}{w})^{-2}## (and then the ##1## cancels with the ##1## in ##( (1-\frac{z}{w})^{-2}) -1 ) ##) in the form ##(1-x)^{-2}##, however this doesn't give me the right answer, can someone please help explain how they got from line ##1## to ##2## ?

Many thanks in advance.