question:
Show that the number of different triples that can be chosen from N items is precisely N(N-1)(N-2)/6.Hint: Use mathematical induction or a counting argument.
answer:
法一:排列组合C(N,3) = N ( N - 1) ( N - 2 ) / ( 3 ! )
= N ( N - 1) ( N - 2 ) / 6
法二:数学归纳法
//高中知识,已经全忘了,而且网上证明也很多,我就不写了