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本文给出组合恒等式C_n~1+2C_N~2+3C_n~3+…+nC_n~n=n·2~(n-1)的六种证法.这个组合恒等式在证明其它组合恒等式和计算组合数的和时常常有用.
In this paper, we give six proofs of the combination identities C_n ~ 1 + 2C_N ~ 2 + 3C_n ~ 3 + ... + nC_n ~ n = n · 2 ~ (n-1) Often and often useful.