Saturday, September 15, 2018

combinatorics - Simplify binomm+n2binomm2binomn2.

Simplify \binom{m+n}{2} - \binom{m}{2} - \binom{n}{2}.






I'm confused on how to approach this problem. I can't think of any counting argument that will help me, and any 1-1 correspondence. All solutions are appreciated.

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