# Cant understand this transition

1. Jan 14, 2010

### nhrock3

http://i50.tinypic.com/2a6w9qg.jpg

cant see how they put the only the denominator to be equal the new expression.
?

2. Jan 14, 2010

### vela

Staff Emeritus
They used

$$\frac{1}{1-x}=1+x+x^2+x^3+...$$

where $x=2z^2/4!+...$. It looks like they made a sign mistake when they did that, though.