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Possible Duplicate:
Combinatorial proof for two identities

is there a combinatorial proof of equation below? (parallel summation for binomials): $$\sum_{k=0}^{n} \binom{n+k-1}{k} = \binom{2n}{n}$$
It seems like it's easy to prove combinatorically,
yet I cannot find proper proof...

3 Answers 3