Mathematics
Senior High

なぜこの式が成り立つのですか?導出の仕方はありますか?

nCr=n-1Cr-1 + n-1 Cr

Answers

右辺=(n-1)!/(r-1)!(n-r)!+(n-1)!/(r)!(n-r-1)!
=[r(n-1)!+(n-r)(n-1)!]/r!(n-r)!
=n!/r!(n-r)!=左辺

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