作者ooopppeeennn (open)
看板Grad-ProbAsk
标题[问题] 两题离散
时间Mon Mar 16 22:21:02 2009
(a)In how many ways can 3n letters be selected from 2n A's,
2n B's, and 2n C's ?
(b)In how many ways can n letters be selected from an unlimited
supply of A's , B's ,and C's if each selection must include
an even number of A's ?
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1F：→ ssccg:(a) C(3n+2,2) - 3C(n,1) (b)求1/(1-x^2)(1-x)^2中x^n的系数 03/16 23:18
2F：推 s987692:请问一下A是怎麽导的??? 03/17 00:57
3F：→ ssccg:相当於a+b+c=3n，0≦a、b、c≦2n，用排容 03/17 05:02
4F：推 s987692:喔! 谢噜! 03/17 19:21
5F：→ ssccg:应该是 C(3n+2,2) - 3C(n+1,2) 才对 03/18 11:32