2010 AMC 12B 第 17 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。

17.

一个 3×33\times3 数组中的项包含数字 1199, 各一次,并且每一行和每一列中的项都按递增顺序排列。 这样的数组有多少个?

The entries in a 3×33\times3 array include all the digits from 11 through 9,9, arranged so that the entries in every row and column are in increasing order. How many such arrays are there?

1818

2424

3636

4242

6060

答案:D
知识点:有限制的排列分类讨论
难度评级:1980
解答:

记第 ii 行第 jj 列的项为 aija_{ij}。题设条件迫使 a11=1a_{11}=1a33=9a_{33}=9,且 a22{4,5,6}a_{22}\in\{4,5,6\}

如果 a22=4a_{22}=4{a12,a21}={2,3}\{a_{12},a_{21}\}=\{2,3\}{5,6,7,8}\{5,6,7,8\} 被分成互补的两对,填入最后一行和最后一列剩下的位置:分法有 (42)=6\binom42=6 种, 再乘 {2,3}\{2,3\}22 种顺序,得到 1212 个数组。由对称性,a22=6a_{22}=6 也给出 1212 个。

如果 a22=5a_{22}=5{a12,a13,a23}\{a_{12},a_{13},a_{23}\}{a21,a31,a32}\{a_{21},a_{31},a_{32}\}{2,3,4,6,7,8}\{2,3,4,6,7,8\} 的互补子集,并受递增顺序限制; 这迫使第一组为 {2,3,4}\{2,3,4\}{6,7,8}\{6,7,8\}; 因此有 (63)2=18\binom63-2=18 个数组。

总共有 12+12+18=4212+12+18=42

因此,正确答案是 D

Write aija_{ij} for the entry in row i,i, column j.j. The conditions force a11=1,a_{11}=1, a33=9,a_{33}=9, and a22{4,5,6}.a_{22}\in\{4,5,6\}.

If a22=4,a_{22}=4, then {a12,a21}={2,3}\{a_{12},a_{21}\}=\{2,3\} and {5,6,7,8}\{5,6,7,8\} split as complementary pairs filling the rest of the last row and column: (42)=6\binom42=6 splits times 22 orders for {2,3}\{2,3\} gives 1212 arrays. By symmetry a22=6a_{22}=6 also gives 12.12.

If a22=5,a_{22}=5, then {a12,a13,a23}\{a_{12},a_{13},a_{23}\} and {a21,a31,a32}\{a_{21},a_{31},a_{32}\} are complementary subsets of {2,3,4,6,7,8}\{2,3,4,6,7,8\} subject to the ordering constraints. The first set can be any three-element subset except {2,3,4}\{2,3,4\} or {6,7,8},\{6,7,8\}, giving (63)2=18\binom63-2=18 arrays.

Altogether 12+12+18=42.12+12+18=42.

Thus, the correct answer is D.

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