Hi all

I have two questions relating to correlation:

Firstly where does the first 'n' in the numerator come from in the far right hand expression of the equation below

http://upload.wikimedia.org/math/c/a...80c9bc4e27.png

Secondly, I am trying to work out the derivation of correlation^2 = r^2. I have started with squaring the above expression and trying to simplify, but I don't feel like I'm going about it the right way. Could someone give me a shove in the right direction please?