2024 AMC 8 第 16 题

先试着解答 2024 AMC 8 第 16 题,然后核对你的答案与视频与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2024 AMC 8 解答,或核对答案

所有题目均经美国数学协会(MAA)官方合法授权使用。

16.

Minh 把 118181 的数字以某种顺序填入一个 9×99 \times 9 网格。她计算每一行和每一列中数字的乘积。最少有多少行和列的乘积能被 33 整除?

Minh enters the numbers 11 through 8181 into the cells of a 9×99 \times 9 grid in some order. She calculates the product of the numbers in each row and column. What is the least number of rows and columns that could have a product divisible by 3?3?

88

99

1010

1111

1212

答案:D
知识点:整除性最优化极端原理
难度评级:1660
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文字解答:

118181 共有 272733 的倍数。一行或一列的乘积能被 33 整除,当且仅当它至少含有一个这样的倍数。

设有 rr 行和 cc 列的乘积能被 33 整除。所有 33 的倍数都必须位于这些 rr 行与 cc 列的交叉格中,所以 2727 个倍数必须装入 rcrc 个格子。若 r+c10r+c\le10,则 rc25rc\le25,格子不够。因此至少要标记 1111 行和列。

可以把 252533 的倍数放入一个 5×55\times5 区块,再把剩下的 22 个放在这些行中的第六列。这样恰好标记 55 行和 66 列,共 1111

正确答案是 D

There are 2727 multiples of 33 from 11 through 8181. A row or column has product divisible by 33 exactly when it contains at least one of these multiples.

Suppose rr rows and cc columns have products divisible by 33. Every multiple of 33 must lie in one of those rr rows and also in one of those cc columns, or else it would create another marked row or column. Thus the 2727 multiples must fit in the rcrc intersection cells. If r+c10r+c\le10, then rc25rc\le25, which is too small. So at least 1111 rows and columns are needed.

This can be done by placing 2525 multiples of 33 in a 5×55\times5 block, then placing the remaining 22 multiples in a sixth column within two of those same rows. Then exactly 55 rows and 66 columns are marked, for a total of 1111.

Thus, D is the correct answer.

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