Limit Process Summation Confusion

[Whalstib]

Hi,

I have been staring at this for a while. I under stand the process and can account for and reproduce every step of this problem EXCEPT I can nit figure out where the 6i/n coms from. I have included the equation and the issue is highlighted in yellow.

Please explain... this is the last piece of the puzzle.

Thanks,

Warren

 

[micromass]

uuuuhm, $(a+b)^2=a^2+2ab+b^2$?

 

[Whalstib]

Man! I am actually getting the calculus but completely blowing the algebra!

Thanks...sigh...

Warren