2017 AMC 12B 第 25 题
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所有题目均经美国数学协会(MAA)官方合法授权使用。
25.
有 个人参加一个在线视频篮球锦标赛。每个人可以属于任意数量的 人球队,但任意两支球队不能有完全相同的 名成员。网站统计显示一个有趣的事实:在所有由这 名参赛者组成的 人子集中,这 人内完整球队数量的平均值,等于在所有由 名参赛者组成的 人子集中,这 人内完整球队数量平均值的倒数。满足 的参赛人数 可以有多少个值?
A set of people participate in an online video basketball tournament. Each person may be a member of any number of -player teams, but no two teams may have exactly the same members. The site statistics show a curious fact: The average, over all subsets of size of the set of participants, of the number of complete teams whose members are among those people is equal to the reciprocal of the average, over all subsets of size of the set of participants, of the number of complete teams whose members are among those people. How many values can be the number of participants?
小提示:
设球队数为 。每支球队在 人子集求和中被计数 次,在 人子集求和中被计数 次
Let be the number of teams. Each team is counted times in the size- sum and times in the size- sum
大提示:
条件化为 ;统计使它为整数的 。
The condition becomes count making this an integer
解答:
设球队数为 。对 人子集求和时,每支球队被计数 次;对 人子集求和时,每支球队被计数 次。两个平均值分别为 和 。令第一个等于第二个的倒数并化简,得到 我们需要它在 时为正整数。令 ;作为五个连续整数的乘积, 总能被 整除。 是 的倍数这一条件对模 的 个余数成立;是 的倍数对模 的 个余数成立;是 的倍数对模 的 个余数成立。因此由中国剩余定理,模 共有 个解。于是 中有 个值;去掉 (它们小于 ),再加上 (因为 ),得到 个有效值。
所以正确答案是 D。
Let be the number of teams. Summing over size- subsets counts each team times and over size- subsets times. The averages are and setting the first equal to the reciprocal of the second and simplifying gives We need this to be a positive integer with Let as a product of five consecutive integers, is always divisible by The condition that is divisible by holds for residues modulo divisibility by holds for residues modulo and divisibility by holds for residues modulo The Chinese Remainder Theorem therefore gives solutions modulo So there are values in removing (which are below ) and adding (since ) gives valid values.
Thus, the correct answer is D.
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