2021 AIME II 第 3 题

先试着解答 2021 AIME II 第 3 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2021 AIME II 解答,或核对答案

所有题目均经美国数学协会(MAA)官方合法授权使用。

3.

求数字 1,2,3,4,51, 2, 3, 4, 5 的排列 x1,x2,x3,x4,x5x_1, x_2, x_3, x_4, x_5 的个数,使得五个乘积之和能被 33 整除。 x1x2x3+x2x3x4+x3x4x5+x4x5x1+x5x1x2 \begin{aligned} &x_1x_2x_3 + x_2x_3x_4 \\ &\quad {}+ x_3x_4x_5 + x_4x_5x_1 \\ &\quad {}+ x_5x_1x_2 \end{aligned}

Find the number of permutations x1,x2,x3,x4,x5x_1, x_2, x_3, x_4, x_5 of numbers 1,2,3,4,51, 2, 3, 4, 5 such that the sum of five products x1x2x3+x2x3x4+x3x4x5+x4x5x1+x5x1x2 \begin{aligned} &x_1x_2x_3 + x_2x_3x_4 \\ &\quad {}+ x_3x_4x_5 + x_4x_5x_1 \\ &\quad {}+ x_5x_1x_2 \end{aligned} is divisible by 3.3.

答案:80
知识点:模运算排列分类讨论
难度评级:2350
解答:

在模 33 下考虑。数值 33 是唯一的 33 的倍数,而五个乘积各覆盖循环相邻的三个位置,所以若 xi=3x_i = 3,恰好有两个乘积避开位置 ii:它们分别覆盖位置 i+1,i+2,i+3i+1, i+2, i+3i+2,i+3,i+4i+2, i+3, i+4(下标模 55)。它们的和为 xi+2xi+3(xi+1+xi+4)x_{i+2}x_{i+3}(x_{i+1} + x_{i+4}),由于 xi+2xi+3x_{i+2}x_{i+3} 不能被 33 整除,条件为 xi+1+xi+40(mod3)x_{i+1} + x_{i+4} \equiv 0 \pmod 3

在剩下的数中,11441\equiv 1,而 22552(mod3)\equiv 2 \pmod 3,所以位置 i+1i+1i+4i+4 必须分别取自两个余数类: 有 222=82 \cdot 2 \cdot 2 = 8 个有序选择。剩下两个数填入位置 i+2i+2i+3i+322 种方式。数值 33 的位置有 55 种选择,所以总数为 582=805 \cdot 8 \cdot 2 = 80

Work modulo 3.3. The value 33 is the only multiple of 3,3, and each of the five products covers three cyclically consecutive positions, so if xi=3x_i = 3 exactly two products avoid position i:i: those covering positions i+1,i+2,i+3i+1, i+2, i+3 and i+2,i+3,i+4i+2, i+3, i+4 (indices mod 55). Their sum is xi+2xi+3(xi+1+xi+4),x_{i+2}x_{i+3}(x_{i+1} + x_{i+4}), and since xi+2xi+3x_{i+2}x_{i+3} is not divisible by 3,3, the condition is xi+1+xi+40(mod3).x_{i+1} + x_{i+4} \equiv 0 \pmod 3.

Among the remaining values, 11 and 44 are 1\equiv 1 while 22 and 55 are 2(mod3),\equiv 2 \pmod 3, so positions i+1i+1 and i+4i+4 must take one value from each class: 222=82 \cdot 2 \cdot 2 = 8 ordered choices. The other two values fill positions i+2i+2 and i+3i+3 in 22 ways. With 55 choices for the position of 3,3, the count is 582=80.5 \cdot 8 \cdot 2 = 80.

← 第 2 题#2
完整试卷

其他年份的第 3 题