Find the number of ways of forming a group of 2k people from n couple, where k,n are elements of N with 2k≤n in each case: i) k couples are in such a group, ii) no couples are included in such a group, iii) at least one couple is included in such a group, and iv) exactly two couples are included in such a group.
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Expert's answer
2011-10-06T11:14:29-0400
i) k couples are in such a groupWe should pick up k couples from n
ii) no couples are included insuch a group at first time we can pick nvariants second- n-2(cannot pick 1 couple one member of whitchis picked) … So ans n(n-2)...(n-2k)/(2k)! iii) at least one couple isincluded in such a group C2k2n-all variants to pick up 2k from 2n n(n-2)..(n-2k)/(2k)! it does not fitus. So ans:C2k2n - n(n-2)..(n-2k)/(2k)! iv) exactly two couples are included in such a group. C2n-so many ways areto pick 2 couples C2k-42n-4-we pick from 2n-4people 2k-4 places in our group
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