2014 AMC 12B 第 18 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。

18.

要把数字 1,2,3,4,51, 2, 3, 4, 5 排成一圈。如果不能对从 111515 的每个 nn,都找到圆上连续的一段数字,使其和为 nn,则称这个排列为排列。只相差旋转或翻折的排列视为相同。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 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

答案:B
知识点:环形排列补集计数分类讨论
难度评级:2150
解答:

任意单个数字可得到从 115.5. 的和。如果某个连续段的和为 n,n,剩余数字也组成一个连续段,其和为 15n,15 - n,所以从 10101414 的和也自动可以得到。因此,一个排列不合格,只可能是因为不能得到 667.7.

如果不能得到 66,通过旋转和翻折可设顺序为 1bc5e.1bc5e. 数对 {b,c}\{b,c\} 不能是 {2,3}\{2,3\}{2,4},\{2,4\},所以 e=2;e=2; 再避开连续段 213213,就迫使排列为 14352.14352. 如果不能得到 77,将顺序写成 2bc5e.2bc5e. 此时 {b,c}\{b,c\} 不能是 {3,4}\{3,4\}{1,4},\{1,4\},所以 e=4,e=4,避开 421421 就迫使排列为 23154.23154.

在旋转和翻折意义下,只有这两个不合格的排列。

因此,正确答案是 B

Any single number covers sums 11 through 5.5. If a consecutive block sums to n,n, the remaining numbers form a consecutive block summing to 15n,15 - n, so sums 1010 through 1414 are automatically covered as well. Thus an arrangement is bad only if it fails to produce 66 or 7.7.

If 66 cannot be formed, rotate and reflect so the order is 1bc5e.1bc5e. The pair {b,c}\{b,c\} cannot be {2,3}\{2,3\} or {2,4},\{2,4\}, so e=2;e=2; avoiding the block 213213 then forces 14352.14352. If 77 cannot be formed, write the order as 2bc5e.2bc5e. Now {b,c}\{b,c\} cannot be {3,4}\{3,4\} or {1,4},\{1,4\}, so e=4,e=4, and avoiding 421421 forces 23154.23154.

These are the only two bad arrangements up to rotation and reflection.

Thus, the correct answer is B.

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