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.

nn 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 55 人球队,但任意两支球队不能有完全相同的 55 名成员。网站统计显示一个有趣的事实:在所有由这 nn 名参赛者组成的 99 人子集中,这 99 人内完整球队数量的平均值,等于在所有由 nn 名参赛者组成的 88 人子集中,这 88 人内完整球队数量平均值的倒数。满足 9n20179 \le n \le 2017 的参赛人数 nn 可以有多少个值?

A set of nn people participate in an online video basketball tournament. Each person may be a member of any number of 55-player teams, but no two teams may have exactly the same 55 members. The site statistics show a curious fact: The average, over all subsets of size 99 of the set of nn participants, of the number of complete teams whose members are among those 99 people is equal to the reciprocal of the average, over all subsets of size 88 of the set of nn participants, of the number of complete teams whose members are among those 88 people. How many values n,n, 9n2017,9 \le n \le 2017, can be the number of participants?

477477

482482

487487

557557

562562

答案:D
知识点:双重计数整除性中国剩余定理
难度评级:2650
小提示:

设球队数为 TT。每支球队在 99 人子集求和中被计数 (n54)\binom{n-5}{4} 次,在 88 人子集求和中被计数 (n53)\binom{n-5}{3}

Let TT be the number of teams. Each team is counted (n54)\binom{n-5}{4} times in the size-99 sum and (n53)\binom{n-5}{3} times in the size-88 sum

大提示:

条件化为 T=T = n(n1)(n2)(n3)(n4)253257\dfrac{n(n-1)(n-2)(n-3)(n-4)}{2^5 \cdot 3^2 \cdot 5 \cdot 7};统计使它为整数的 nn

The condition becomes T=T = n(n1)(n2)(n3)(n4)253257;\dfrac{n(n-1)(n-2)(n-3)(n-4)}{2^5 \cdot 3^2 \cdot 5 \cdot 7}; count nn making this an integer

解答:

设球队数为 TT。对 99 人子集求和时,每支球队被计数 (n54)\binom{n-5}{4} 次;对 88 人子集求和时,每支球队被计数 (n53)\binom{n-5}{3} 次。两个平均值分别为 (n54)T(n9)\dfrac{\binom{n-5}{4}T}{\binom n9}(n53)T(n8)\dfrac{\binom{n-5}{3}T}{\binom n8}。令第一个等于第二个的倒数并化简,得到 T=n(n1)(n2)(n3)(n4)253257 \begin{aligned} &T \\ &\quad {}= \scriptsize \frac{n(n-1)(n-2)(n-3)(n-4)}{2^5 \cdot 3^2 \cdot 5 \cdot 7} \end{aligned}\text{。} 我们需要它在 n9n \ge 9 时为正整数。令 N=N = n(n1)(n2)(n3)(n4)n(n-1)(n-2)(n-3)(n-4);作为五个连续整数的乘积,NN 总能被 55 整除。NN77 的倍数这一条件对模 7755 个余数成立;是 99 的倍数对模 9977 个余数成立;是 3232 的倍数对模 161688 个余数成立。因此由中国剩余定理,模 lcm(7,9,16)=1008\operatorname{lcm}(7,9,16)=1008 共有 578=2805\cdot7\cdot8=280 个解。于是 1n20161 \le n \le 2016 中有 560560 个值;去掉 n=1,2,3,4n = 1, 2, 3, 4(它们小于 99),再加上 n=2017n = 2017(因为 20171(mod1008)2017 \equiv 1 \pmod{1008}),得到 5604+1=557560 - 4 + 1 = 557 个有效值。

所以正确答案是 D

Let TT be the number of teams. Summing over size-99 subsets counts each team (n54)\binom{n-5}{4} times and over size-88 subsets (n53)\binom{n-5}{3} times. The averages are (n54)T(n9)\dfrac{\binom{n-5}{4}T}{\binom n9} and (n53)T(n8);\dfrac{\binom{n-5}{3}T}{\binom n8}; setting the first equal to the reciprocal of the second and simplifying gives T=n(n1)(n2)(n3)(n4)253257. \begin{aligned} &T \\ &\quad {}= \scriptsize \frac{n(n-1)(n-2)(n-3)(n-4)}{2^5 \cdot 3^2 \cdot 5 \cdot 7}. \end{aligned} We need this to be a positive integer with n9.n \ge 9. Let N=N = n(n1)(n2)(n3)(n4);n(n-1)(n-2)(n-3)(n-4); as a product of five consecutive integers, NN is always divisible by 5.5. The condition that NN is divisible by 77 holds for 55 residues modulo 7;7; divisibility by 99 holds for 77 residues modulo 9;9; and divisibility by 3232 holds for 88 residues modulo 16.16. The Chinese Remainder Theorem therefore gives 578=2805\cdot7\cdot8=280 solutions modulo lcm(7,9,16)=1008.\operatorname{lcm}(7,9,16)=1008. So there are 560560 values in 1n2016;1 \le n \le 2016; removing n=1,2,3,4n = 1, 2, 3, 4 (which are below 99) and adding n=2017n = 2017 (since 20171(mod1008)2017 \equiv 1 \pmod{1008}) gives 5604+1=557560 - 4 + 1 = 557 valid values.

Thus, the correct answer is D.

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