2020 AMC 8 第 24 题

先试着解答 2020 AMC 8 第 24 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 2020 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。

24.

一个大正方形区域铺有 n2n^2 块阴影正方形瓷砖,每块边长为 ss 英寸。每块瓷砖周围有宽 dd 英寸的边框。下图显示了 n=3n=3 的情况。当 n=24n=24 时,576576 块阴影瓷砖覆盖了大正方形区域面积的 64%64\%。当 nn 取这个较大的值时,ds\frac{d}{s} 是多少?
A large square region is paved with n2n^2 shaded square tiles, each measuring ss inches on a side. A border dd inches wide surrounds each tile. The figure below shows the case for n=3.n=3. When n=24,n=24, the 576576 shaded tiles cover 64%64\% of the area of the large square region. What is the ratio ds\frac{d}{s} for this larger value of n?n?
625\dfrac{6}{25}
14\dfrac{1}{4}
925\dfrac{9}{25}
716\dfrac{7}{16}
916\dfrac{9}{16}
答案:A
知识点:面积比正方形(几何)
小提示:
大正方形边长是 24s+25d24s+25d
The large side length is 24s+25d24s+25d
大提示:
比较阴影面积 242s224^2s^2 与总面积
Compare shaded area 242s224^2s^2 with total area
视频讲解:
解答视频缩略图
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文字解答:
当 n=24n=24 时,阴影瓷砖总面积为 242s224^2s^2。大正方形一边有 2424 块瓷砖和 2525 条边框,所以边长为 24s+25d24s+25d。
阴影部分占 64%=162564\%=\dfrac{16}{25},所以 242s2(24s+25d)2=1625。 \dfrac{24^2s^2}{(24s+25d)^2}=\dfrac{16}{25}\text{。} 取正平方根得 24s24s+25d=45\dfrac{24s}{24s+25d}=\dfrac{4}{5}。
因此 120s=96s+100d120s=96s+100d,所以 24s=100d24s=100d,ds=625\dfrac{d}{s}=\dfrac{6}{25}。
正确答案是 A。
For n=24,n=24, the shaded tile area is 242s2.24^2s^2. Each side of the large square consists of 2424 tiles and 2525 borders, so its side length is 24s+25d.24s+25d.
The shaded tiles cover 64%=162564\%=\dfrac{16}{25} of the large square, so 242s2(24s+25d)2=1625. \dfrac{24^2s^2}{(24s+25d)^2}=\dfrac{16}{25}. Taking positive square roots gives 24s24s+25d=45.\dfrac{24s}{24s+25d}=\dfrac{4}{5}.
Thus 120s=96s+100d,120s=96s+100d, so 24s=100d24s=100d and ds=625.\dfrac{d}{s}=\dfrac{6}{25}.
Thus, the correct answer is A.
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