1993 AMC 8 第 25 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。

25.

一个棋盘由边长为一英寸的正方形组成。一张边长为 1.51.5 英寸的正方形卡片放在棋盘上,使它覆盖了 nn 个小正方形的部分或全部面积。nn 的最大可能值是

A checkerboard consists of one-inch squares. A square card, 1.51.5 inches on a side, is placed on the board so that it covers part or all of the area of each of nn squares. The maximum possible value of nn is

4455

44 or 55

6677

66 or 77

8899

88 or 99

10101111

1010 or 1111

1212 或更多

1212 or more

答案:E
知识点:最优化勾股定理
难度评级:1270
解答:

将卡片旋转 4545^\circ,并把中心放在四个棋盘小格相交的角点上。因为卡片对角线 1.52+1.52=4.52.1\sqrt{1.5^2 + 1.5^2} = \sqrt{4.5} \approx 2.1 大于 22,卡片的四个角都会越过网格线伸入下一格。

卡片覆盖中央的 2×22 \times 244 个小正方形,并且在四边各伸入另外 22 个小正方形,总共 4+4×2=124 + 4 \times 2 = 12 个。

这也是最大可能值。卡片边长只有 1.51.5 英寸,所以水平和竖直方向跨度都最多约 2.12.1 英寸,因此它位于一个 4×44 \times 4、共含 1616 格的方格块内。只有卡片的四个尖角能到达该方格块边缘,它不可能到达这个方格块的四个角格,所以最多覆盖 1212 个。既然 1212 可以达到,最大值就是 1212,属于“1212 或更多”。

所以正确答案是 E

Tilt the card 4545^\circ and center it on a corner where four grid squares meet, as shown. Because the card's diagonal, 1.52+1.52=4.52.1,\sqrt{1.5^2 + 1.5^2} = \sqrt{4.5} \approx 2.1, is longer than 2,2, each of the four corners of the card reaches past a grid line into the next square.

The card covers the central 2×22 \times 2 block of 44 squares and pokes into 22 more squares on each of its four sides, giving 4+4×2=124 + 4 \times 2 = 12 squares.

This is also the most possible. The card is only 1.51.5 inches wide, so its overall width and height are each at most 2.12.1 inches; it therefore lies within a 4×44 \times 4 block of 1616 squares. Its four pointed corners are the only parts that reach the edge of that block, so it can never reach the four corner squares of the block, leaving at most 12.12. Since 1212 is achievable, the maximum is 12,12, which falls in the range "1212 or more."

Thus, the correct answer is E .

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