2014 AMC 10B 第 24 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。

24.

数字 1,2,3,4,51, 2, 3, 4, 5 要排成一个圆。若并非对每个 111515nn,都能找到圆上一段连续出现的数字使其和为 nn,则称这种排列为 bad\textit{bad}。只相差旋转或翻转的排列视为相同。有多少种不同的坏排列?

The numbers 1,2,3,4,51, 2, 3, 4, 5 are to be arranged in a circle. An arrangement is bad\textit{bad} if it is not true that for every nn from 11 to 1515 one can find a subset of the numbers that appear consecutively on the circle that sum to n.n. Arrangements that differ only by a rotation or a reflection are considered the same. How many different bad arrangements are there?

11

22

33

44

55

5. 5 .

答案:B
知识点:环形排列分类讨论
难度评级:2390
解答:

单个数字给出 1155,它们的补集给出 10101414,全部五个数给出 1515。因此只需要检查能否得到和 6677

若无法得到和 66,则 11 不与 55 相邻。通过旋转和翻转,可写成 1bc5e1bc5e。相邻块 bcbc 不能是 {2,3}\{2,3\}{2,4}\{2,4\},因为 1+2+3=61+2+3=62+4=62+4=6。于是 e=2e=2,再避免连续块 2,1,32,1,3,得到坏排列 1435214352

若无法得到 77,则 22 不与 55 相邻,可写成 2bc5e2bc5e。此时 bcbc 不能是 {3,4}\{3,4\}{1,4}\{1,4\},所以 e=4e=4。再避免连续块 4,2,14,2,1,得到 b=3, c=1b=3,\ c=1,即坏排列 2315423154

这两个排列确实都是坏排列,分别无法得到和 66 与和 77。因此共有 22 种坏排列。

所以正确答案是 B

Single numbers give sums 11 through 55, complements give sums 1010 through 1414, and all five numbers give 1515. So an arrangement is good exactly when consecutive blocks can make sums 66 and 77.

If sum 66 is impossible, then 11 is not adjacent to 55. By rotating and reflecting, write the arrangement as 1bc5e1bc5e. The adjacent pair bcbc cannot be {2,3}\{2,3\} or {2,4}\{2,4\}, since 1+2+3=61+2+3=6 and 2+4=62+4=6. Thus e=2e=2, and avoiding the consecutive block 2,1,32,1,3 forces the bad arrangement 1435214352.

If sum 77 is impossible, then 22 is not adjacent to 55. Similarly write the arrangement as 2bc5e2bc5e. Now bcbc cannot be {3,4}\{3,4\} or {1,4}\{1,4\}, so e=4e=4. To avoid the consecutive block 4,2,14,2,1, the remaining order must be b=3, c=1b=3,\ c=1, giving 2315423154.

These two arrangements are indeed bad, one missing sum 66 and the other missing sum 77. Hence there are 22 bad arrangements.

Thus, the correct answer is B .

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