2010 AIME II 第 11 题

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11.

定义一个 T-grid 为满足以下两个性质的 3×33 \times 3 矩阵:
(1)(1) 恰好五个元素为 11,剩下四个元素为 00。
(2)(2) 在八条行、列和长对角线中(长对角线为 {a13,a22,a31}\{a_{13}, a_{22}, a_{31}\} 和 {a11,a22,a33}\{a_{11}, a_{22}, a_{33}\}),至多有一条的三个元素全相等。
求不同 T-grid 的个数。
Define a T-grid to be a 3×33 \times 3 matrix which satisfies the following two properties:
(1)(1) Exactly five of the entries are 11’s, and the remaining four entries are 00’s.
(2)(2) Among the eight rows, columns, and long diagonals (the long diagonals are {a13,a22,a31}\{a_{13}, a_{22}, a_{31}\} and {a11,a22,a33}\{a_{11}, a_{22}, a_{33}\}), no more than one of the eight has all three entries equal.
Find the number of distinct T-grids.
答案:68
知识点:补集计数分类讨论
难度评级:3060
小提示:
满足第一条性质的 11 的放置共有 (95)=126\binom{9}{5} = 126 种;减去其中有两条或更多条全同线的情况。
There are (95)=126\binom{9}{5} = 126 placements of the 11’s; subtract those where two or more of the eight lines are constant
大提示:
两条全同线必须要么是一条全 11 的行或列和一条与它平行的全 00 线,要么是两条相交的全 11 线,后者恰好使用五个 11。
Two constant lines must be either a row/column of 11’s with a parallel line of 00’s, or two crossing lines of 11’s, which use exactly five 11’s
解答:
满足 (1)(1) 的矩阵有 (95)=126\binom{9}{5} = 126 个;我们减去有两条或更多全同线的矩阵。两条全 00 线不可能(它们至少需要 55 个零),一条全 11 线和一条全 00 线不能相交,因而必须是平行行或平行列;同样,两条全 11 线不能平行(会有 66 个一),所以必须相交,使用恰好 3+3−1=53 + 3 - 1 = 5 个一。
情况 11:一条全 11 线和一条与它平行的全 00 线。全 11 的行或列有 66 种选择,平行的全 00 线有 22 种选择,剩下的平行线用两个 11 和一个 00 填充,有 33 种方法,共 6⋅2⋅3=366 \cdot 2 \cdot 3 = 36 个矩阵。每条垂直线随后都同时含有 11 和 00,所以不会出现第三条全同线,也没有重复计数。
情况 22:两条相交的全 11 线,其余位置为 00。这对线可以是一行一列(3⋅3=93 \cdot 3 = 9),一行或一列与一条对角线(6⋅2=126 \cdot 2 = 12),或两条对角线(11),共 2222 个矩阵;可检查其余四个 00 从不会形成一条全同线。所以答案为 126−36−22=68126 - 36 - 22 = 68。
There are (95)=126\binom{9}{5} = 126 matrices satisfying (1);(1); we subtract those with two or more constant lines. Two lines of 00’s are impossible (they would need at least 55 zeros), and a line of 11’s and a line of 00’s cannot cross, so they must be parallel rows or parallel columns; likewise two lines of 11’s cannot be parallel (66 ones), so they must cross, using exactly 3+3−1=53 + 3 - 1 = 5 ones.
Case 1:1: a line of 11’s and a parallel line of 00’s. There are 66 choices for the all-11 row or column, 22 for the parallel all-00 line, and 33 ways to fill the remaining parallel line with two 11’s and one 0:0: 6⋅2⋅3=366 \cdot 2 \cdot 3 = 36 matrices. Every perpendicular line then contains both a 11 and a 0,0, so no third constant line appears and nothing is double-counted.
Case 2:2: two crossing lines of 11’s and 00’s elsewhere. The pair can be a row and a column (3⋅3=93 \cdot 3 = 9), a row or column with a diagonal (6⋅2=126 \cdot 2 = 12), or the two diagonals (11), for 2222 matrices; one checks the four remaining 00’s never form a constant line. So 126−36−22=68.126 - 36 - 22 = 68.
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