2017 AMC 8 第 16 题

先试着解答 2017 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 2017 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.

在下图中,在边 BCBC 上选择点 DD,使得 △ACD\triangle ACD 和 △ABD\triangle ABD 的周长相等。△ABD\triangle ABD 的面积是多少?
In the figure shown below, choose point DD on side BCBC so that △ACD\triangle ACD and △ABD\triangle ABD have equal perimeters. What is the area of △ABD?\triangle ABD?
34\dfrac{3}{4}
32\dfrac{3}{2}
22
125\dfrac{12}{5}
52\dfrac{5}{2}
答案:D
知识点:面积比周长
小提示:
用周长相等条件求 BDBD 和 CDCD
Use the equal-perimeter condition to find BDBD and CDCD.
大提示:
一旦 BCBC 被分割,两个三角形从 AA 引出的高相同,所以面积按底边长度比较
Once BCBC is split, the two triangles have the same altitude from AA, so compare areas by base lengths.
视频讲解:
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文字解答:
要使两个三角形周长相等,BC‾\overline{BC} 必须分成 CD‾=3\overline{CD} = 3 和 BD‾=2\overline{BD} = 2。
△ACD\triangle ACD 和 △ABD\triangle ABD 有相同的高,所以面积与底边长度成比例。这说明 △ABD\triangle ABD 的面积是 25\dfrac{2}{5} 倍的 △ABC\triangle ABC 的面积,即 25⋅3⋅42=125。\dfrac{2}{5} \cdot 3 \cdot \dfrac{4}{2} = \dfrac{12}{5}\text{。}
所以正确答案是 D。
The only way to split BC‾\overline{BC} into two parts such that the two triangles have the same perimeter is if CD‾=3\overline{CD} = 3 and BD‾=2.\overline{BD} = 2.
△ACD\triangle ACD and △ABD\triangle ABD have the same altitudes, so their areas are proportional to their bases. This means that the area of △ABD\triangle ABD is 25\dfrac{2}{5} the area of △ABC,\triangle ABC, which is 25⋅3⋅42=125.\dfrac{2}{5} \cdot 3 \cdot \dfrac{4}{2} = \dfrac{12}{5}.
Thus, D is the correct answer.
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