Has anyone managed to do the combinatorial proof for this one? I am completely stuck. I have the vague idea that it is related to the binomial theorem, but all of my reasoning stalls after expressing (x+y)^2n = (x+y)^n*(x+y)^n. I could be going in a dead end, of course. Any suggestions?
--Jberlako 16:38, 4 February 2009 (UTC)
