How does this transition work?

[metalhead]

Hi there
I have been stumped by this (probably simple) math transition in the attached file. Can someone please explain it in full detail?
Big thanks http://postimage.org/image/11vseawf8/"

<< edit by berkeman to post the explicit URL >>

http://postimage.org/image/11vseawf8

 

[micromass]

I think you forgot to attach a file

 

[gsal]

You mean, explain what they did? as in they allowed i to take the values 0, 1, and 2 and explicitly wrote the terms of the summation?