2 A centain law firm consist of 4 senior partners and 6 junor partners. How many different groups of 3 partners can be formed in which at least one member of the group is a senior partner?(Two groups are considered different if at least one group member is different.) A 48 B 100 C 120 D 288 E 600 第二题应该选B C(3,10) - C(3,6) = 120 - 20 = 100 至少一个是senior,用所有可能的组合减去没有senior的组合。 也可以C(4,1)C(6,2)+C(4,2)C(6,1)+C(4,3) = 4*15 + 6*6 + 4 = 100 含一个senior的组合 + 含两个senior的组合 + 全都是senior的组合
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