2014 AMC 10B 第 24 题
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24.
数字 要排成一个圆。若并非对每个 到 的 ,都能找到圆上一段连续出现的数字使其和为 ,则称这种排列为 。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 are to be arranged in a circle. An arrangement is if it is not true that for every from to one can find a subset of the numbers that appear consecutively on the circle that sum to Arrangements that differ only by a rotation or a reflection are considered the same. How many different bad arrangements are there?
答案:B
解答:
单个数字给出 到 ,它们的补集给出 到 ,全部五个数给出 。因此只需要检查能否得到和 与 。
若无法得到和 ,则 不与 相邻。通过旋转和翻转,可写成 。相邻块 不能是 或 ,因为 且 。于是 ,再避免连续块 ,得到坏排列 。
若无法得到 ,则 不与 相邻,可写成 。此时 不能是 或 ,所以 。再避免连续块 ,得到 ,即坏排列 。
这两个排列确实都是坏排列,分别无法得到和 与和 。因此共有 种坏排列。
所以正确答案是 B。
Single numbers give sums through , complements give sums through , and all five numbers give . So an arrangement is good exactly when consecutive blocks can make sums and .
If sum is impossible, then is not adjacent to . By rotating and reflecting, write the arrangement as . The adjacent pair cannot be or , since and . Thus , and avoiding the consecutive block forces the bad arrangement .
If sum is impossible, then is not adjacent to . Similarly write the arrangement as . Now cannot be or , so . To avoid the consecutive block , the remaining order must be , giving .
These two arrangements are indeed bad, one missing sum and the other missing sum . Hence there are bad arrangements.
Thus, the correct answer is B .
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