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For questions that explicitly reference the binomial coefficients, Pascal's Triangle, and Binomial identities.

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How to prove the combinatorial equality? [closed]

Please, help me to understand following convolution (or give a reference): $$ \sum_{R=0}^N \binom{R}{r} \binom{N-R}{n-r} = \binom{N+1}{n+1} $$ Why is it true? Thank you!