$$\binom{r}{r}+\binom{r+1}{r}+\binom{r+2}{r}+...+\binom{n}{r}=\binom{n+1}{r+1}$$
I know how to prove this by induction.
But I got stuck with proving it combinatorially.
How do I start? and please give some directions.
2026-05-05 05:43:33.1777959813
how do I prove this formula combinatorially?
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