Combinatorial proofs of the identity $(a+b)^2 = a^2 +b^2 +2ab$ The question I have is to give a combinatorial proof of the identity $(a+b)^2 = a^2 +b^2 +2ab$.  
I understand the concept of combinatorial proofs but am having some trouble getting started with this problem, any help would be appreciated. 

Draw a square of side $a+b$ and a line parallel to each pair of sides.  Where should you place the line?
Imagine this broken into a checkerboard:

