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7个相同的小球.任意放入四个不同的盒子中.每个盒子都不空的放法共有20种.
7个相同的小球.任意放入四个不同的盒子中.每个盒子都不空的放法共有20种.
发布时间:2021-02-22 09:46:18
17、7个相同的小球,任意放入四个不同的盒子中,每个盒子都不空的放法共有20
种.
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答案:
分析:
用4块挡板把7个小球分成4份,每一份至少有一个,根据7个球有6个空,任选其中3个空,分成4份,问题就转化成从6个元素中选3个的组合数.
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已知{an}是等比数列.且an>0.a2a4+2a3a5+a4a6=25.那么a3+a5的
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