2019 AMC 10A 第 20 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。

20.

数字 1,2,,91,2,\dots,9 被随机放入 3×33 \times 3 方格的 99 个格子中。每格一个数,每个数用一次。每一行和每一列的数字和都是奇数的概率是多少?

The numbers 1,2,,91,2,\dots,9 are randomly placed into the 99 squares of a 3×33 \times 3 grid. Each square gets one number, and each of the numbers is used once. What is the probability that the sum of the numbers in each row and each column is odd?

121\dfrac{1}{21}

114\dfrac{1}{14}

563\dfrac{5}{63}

221\dfrac{2}{21}

17\dfrac{1}{7}

答案:B
知识点:奇偶性基本概率
难度评级:1820
解答:

行或列的和为奇数,当且仅当其中有 00 个或 22 个偶数。

要满足这一点,44 个偶数所在的格子必须组成一个边平行于大正方形的矩形。

理解这一点的方法是:先选一个偶数所在格子,然后需要在同一行选另一个偶数格 xx,在同一列选另一个偶数格 yy

最后一个偶数必须与 xx 同列、与 yy 同行,于是形成上述矩形。

这样的矩形有四个 2×22 \times 2、两个 3×23 \times 2、两个 2×32 \times 3、一个 3×33 \times 3

因此偶数位置共有 种。 4+2+2+1=94+2+2+1=9

偶数有 4!4! 种排列,奇数有 5!5! 种排列。

所以满足条件的排列共有 种。 94!5! 9 \cdot 4! \cdot 5!

所有排列共有 9!9! 种,因此所求概率为 94!5!9!=114. \dfrac{9 \cdot 4! \cdot 5!}{9!} = \dfrac{1}{14}.

所以正确答案是 B

Note that the only way to get an odd sum is if there are either 00 or 22 even numbers in the row or column.

The only way for this to happen is if the 44 even numbers form a rectangle with sides parallel to the large square.

The way to see this is we choose a spot for the first even number. Then we need to choose another square in the same row, x,x, and column, y,y, to be even.

The final even has to be in same column as xx and the same row as y.y. This forms the aforementioned rectangle.

There are four 2×22 \times 2 rectangles, two 3×23 \times 2 rectangles, two 2×32 \times 3 rectangles, and one 3×33 \times 3 rectangle.

This gives a total of 4+2+2+1=94+2+2+1=9 possible sets of positions for the even numbers.

There are 4!4! ways to arrange the even numbers and 5!5! ways to arrange the odd numbers.

This means that there are a total of 94!5! 9 \cdot 4! \cdot 5! configurations of squares that satisfy the condition.

There are a total of 9!9! arrangements with no restrictions. The probability is therefore 94!5!9!=114. \dfrac{9 \cdot 4! \cdot 5!}{9!} = \dfrac{1}{14}.

Thus, B is the correct answer.

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