2024 AMC 8 第 23 题

先试着解答 2024 AMC 8 第 23 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。

23.

Rodrigo 有一张很大的方格纸。首先,他画一条线段连接点 (0,4)(0, 4) 与点 (2,0)(2, 0),并给线段内部穿过的 44 个方格涂色,如下图所示。接着,Rodrigo 画一条线段连接点 (2000,3000)(2000, 3000) 与点 (5000,8000)(5000, 8000)。他同样给线段内部穿过的方格涂色。这次他会涂多少个方格?
Rodrigo has a very large piece of graph paper. First he draws a line segment connecting point (0,4)(0, 4) to point (2,0)(2, 0) and colors the 44 cells whose interiors intersect the segment, as shown below. Next, Rodrigo draws a line segment connecting point (2000,3000)(2000, 3000) to point (5000,8000).(5000, 8000). Again he colors the cells whose interiors intersect the segment. How many cells will he color this time?
60006000
65006500
70007000
75007500
80008000
答案:C
知识点:最大公约数格点
小提示:
对端点坐标差为 aa 和 bb 的线段,数它穿过的格子
For a segment with integer endpoint differences aa and bb, count grid cells crossed
大提示:
使用 a+b−gcd⁡(a,b)a+b-\gcd(a,b) 计算内部被线段穿过的方格数
Use a+b−gcd⁡(a,b)a+b-\gcd(a,b) for cells whose interiors are intersected
视频讲解:
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文字解答:
若线段水平差为 aa、竖直差为 bb,它穿过 aa 条竖直网格线和 bb 条水平网格线,但在格点处的穿越会被重复计算。因此内部被线段穿过的方格数为 a+b−gcd⁡(a,b)。 a+b-\gcd(a,b)\text{。}
这里坐标差为 30003000 和 50005000,且 gcd⁡(3000,5000)=1000\gcd(3000,5000)=1000。所以方格数为 3000+5000−1000=7000。 3000+5000-1000=7000\text{。}
正确答案是 C。
For a segment whose endpoint differences are aa horizontally and bb vertically, the segment crosses aa vertical grid lines and bb horizontal grid lines, but crossings at lattice points are counted twice. Therefore the number of cells whose interiors are intersected is a+b−gcd⁡(a,b). a+b-\gcd(a,b).
Here the endpoint differences are 30003000 and 50005000, and gcd⁡(3000,5000)=1000\gcd(3000,5000)=1000. Thus the number of cells colored is 3000+5000−1000=7000. 3000+5000-1000=7000.
Thus, C is the correct answer.
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