2025 AIME I 第 5 题

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5.

8!=403208! = 40320 个八位正整数,它们恰好各使用数字 1,2,3,4,5,6,7,81, 2, 3, 4, 5, 6, 7, 8 一次。 令 NN 为这些整数中能被 2222 整除的个数。求 NN20252025 的差。

There are 8!=403208! = 40320 eight-digit positive integers that use each of the digits 1,2,3,4,5,6,7,81, 2, 3, 4, 5, 6, 7, 8 exactly once. Let NN be the number of these integers that are divisible by 22.22. Find the difference between NN and 2025.2025.

答案:279
知识点:整除性排列分类讨论
难度评级:2510
解答:

所有数字之和为 36361111 整除要求数字交错和是 1111 的倍数,所以若奇数位上的四个数字和为 aa,则 a(36a)=2a36a - (36 - a) = 2a - 36 必须是 1111 的倍数。因为 10a2610 \le a \le 26,唯一可能是 a=18a = 18:每组四个位置的数字和都为 1818{1,,8}\{1, \ldots, 8\} 中和为 1818 的四元素子集为 一共有八个,并且两两互为补集。 {1,2,7,8}, {1,3,6,8}, {1,4,5,8}, {1,4,6,7}, {2,3,5,8}, {2,3,6,7}, {2,4,5,7}, {3,4,5,6}, \begin{gathered} \{1,2,7,8\},\ \{1,3,6,8\},\ \\ \{1,4,5,8\},\ \{1,4,6,7\},\ \\ \{2,3,5,8\},\ \{2,3,6,7\},\ \\ \{2,4,5,7\},\ \{3,4,5,6\}, \end{gathered}

选择这 88 个子集中的哪一个占据偶数位,其中包含个位;其补集占据奇数位。若该子集含有 kk 个偶数数字,则个位数字有 kk 种选择,其余偶数位有 3!3! 种排列,奇数位有 4!4! 种排列, 共 144k144k 个数。互补子集的 kk 值之和为 44,所以在全部 88 种选择中 k=16\sum k = 16。因此 N=14416=2304N = 144 \cdot 16 = 2304,且 N2025=279N - 2025 = 279

The digits sum to 36.36. Divisibility by 1111 requires the alternating sum of digits to be a multiple of 11,11, so if the four digits in odd positions sum to a,a, then a(36a)=2a36a - (36 - a) = 2a - 36 must be a multiple of 11.11. Since 10a26,10 \le a \le 26, the only possibility is a=18:a = 18: each block of four positions carries digit sum 18.18. The four-element subsets of {1,,8}\{1, \ldots, 8\} with sum 1818 are {1,2,7,8}, {1,3,6,8}, {1,4,5,8}, {1,4,6,7}, {2,3,5,8}, {2,3,6,7}, {2,4,5,7}, {3,4,5,6}, \begin{gathered} \{1,2,7,8\},\ \{1,3,6,8\},\ \\ \{1,4,5,8\},\ \{1,4,6,7\},\ \\ \{2,3,5,8\},\ \{2,3,6,7\},\ \\ \{2,4,5,7\},\ \{3,4,5,6\}, \end{gathered} eight in all, and they come in complementary pairs.

Choose which of the 88 subsets occupies the even positions (which include the units place); the complement fills the odd positions. If that subset contains kk of the even digits, then the units digit can be chosen in kk ways, the rest of the even positions in 3!3! ways, and the odd positions in 4!4! ways, for 144k144k numbers. Complementary subsets have kk-values summing to 4,4, so over all 88 choices k=16.\sum k = 16. Hence N=14416=2304,N = 144 \cdot 16 = 2304, and N2025=279.N - 2025 = 279.

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