2010 AMC 10B 第 23 题

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23.

一个 3×33 \times 3 数组中填入数字 1199,每个数字恰好使用一次,并且每一行和每一列中的数字都按递增顺序排列。这样的数组有多少个?

The entries in a 3×33 \times 3 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:2030
解答:

aija_{ij} 表示第 ii 行、第 j.j. 列的数。递增条件迫使 a11=1,a_{11}=1, a33=9,a_{33}=9,a22a_{22} 只能是 4,5,4,5,6.6.

a22=4,a_{22}=4,{a12,a21}={2,3}.\{a_{12},a_{21}\}=\{2,3\}.5,6,7,85,6,7,8 中选两个放入左下方的一对位置 {a31,a32};\{a_{31},a_{32}\};另外两个放入右上方的一对位置 {a13,a23}.\{a_{13},a_{23}\}. 每一对都只有一种递增排列。因此共有 2(42)=122\binom42=12 个数阵:左上角周围的 2,32,3 有两种次序,左下方的一对有 (42)\binom42 种选择。把各数反向并旋转数阵,同样可得中心为 6.6.1212 个数阵。

a22=5,a_{22}=5,选择位置 a12,a13,a23.a_{12},a_{13},a_{23}. 中的三个数。它们可以是 {2,3,4,6,7,8}\{2,3,4,6,7,8\} 的任意三元子集,但不能是 {2,3,4}\{2,3,4\}{6,7,8};\{6,7,8\};因为这两种选择会把三个小数或三个大数全放在中心的一侧,从而违反与中心数的必要大小关系。其余每一种选择都唯一确定剩余各数及其递增次序。因此共有 (63)2=18\binom63-2=18 个数阵。

总数为 12+18+12=42.12+18+12=42.

所以正确答案是 D

Let aija_{ij} be the entry in row ii and column j.j. The increasing conditions force a11=1,a_{11}=1, a33=9,a_{33}=9, and a22a_{22} to be 4,5,4,5, or 6.6.

If a22=4,a_{22}=4, then {a12,a21}={2,3}.\{a_{12},a_{21}\}=\{2,3\}. Choose which two of 5,6,7,85,6,7,8 go in the bottom-left pair {a31,a32};\{a_{31},a_{32}\}; the other two go in the top-right pair {a13,a23}.\{a_{13},a_{23}\}. Each pair then has only one increasing order. There are 2(42)=122\binom42=12 arrays: two orders for 2,32,3 around the upper-left corner and (42)\binom42 choices for the bottom-left pair. By reversing the digits and rotating the array, there are also 1212 arrays with center 6.6.

If a22=5,a_{22}=5, choose the three entries in positions a12,a13,a23.a_{12},a_{13},a_{23}. They can be any three of {2,3,4,6,7,8}\{2,3,4,6,7,8\} except {2,3,4}\{2,3,4\} or {6,7,8};\{6,7,8\}; those two choices would put all three small or all three large entries on one side and violate a required comparison with the center. Every other choice uniquely determines the remaining entries and their increasing orders. This gives (63)2=18\binom63-2=18 arrays.

The total is 12+18+12=42.12+18+12=42.

Thus, D is the correct answer.

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