2010 AMC 10B 第 20 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。

20.

两个圆位于正六边形 ABCDEFABCDEF 外。第一个圆与 AB\overline{AB} 相切,第二个圆与 DE\overline{DE} 相切。两个圆都与直线 BCBCFAFA 相切。第二个圆面积与第一个圆面积的比是多少?

Two circles lie outside regular hexagon ABCDEF.ABCDEF. The first is tangent to AB,\overline{AB}, and the second is tangent to DE.\overline{DE}. Both are tangent to lines BCBC and FA.FA. What is the ratio of the area of the second circle to that of the first circle?

1818

2727

3636

8181

108108

答案:D
知识点:正多边形切线特殊直角三角形面积比
难度评级:1960
解答:

参考下图:

设正六边形边长为 11。较小圆内切于一个边长为 11 的等边三角形。

这个等边三角形的内切圆半径为 36\dfrac{\sqrt3}{6}。面积为 π(36)2=π12. \pi \cdot \left(\dfrac{\sqrt3}{6}\right)^2 = \dfrac{\pi}{12}.

设较大圆圆心为 OO,从 OOGH\overline{GH} 作垂线,垂足为 JJ。并连接 OG\overline{OG}

OJG\triangle OJG 是直角三角形,且由 HGI=60\angle HGI = 60^{\circ} 可知,OJG\triangle OJG30609030-60-90 三角形。

OJ=rOJ = r。由三十、六十、九十度三角形关系,OG=2rOG = 2r。接下来用 OGOG 表示较大圆的位置。

于是 OG=32+3+r. OG = \dfrac{\sqrt3}{2} + \sqrt3 + r.

代入 OGOG,得 化简得 2r=32+3+r. 2r = \dfrac{\sqrt3}{2} + \sqrt3 + r. r=332. r = \dfrac{3\sqrt3}{2}.

较大圆面积为 π(332)2=274π. \pi \cdot \left(\dfrac{3\sqrt3}{2}\right)^2 = \dfrac{27}{4}\pi.

所求比值为 27π4π12=81. \dfrac{\frac{27\pi}{4}}{\frac{\pi}{12}} = 81.

所以正确答案是 D

Consider the following diagram:

Assume the regular hexagon has side length 1.1. The smaller circle is inscribed in an equilateral triangle of side length 1.1.

The inradius of this equilateral triangle is 36.\dfrac{\sqrt3}{6}. The area of the circle is then π(36)2=π12. \pi \cdot \left(\dfrac{\sqrt3}{6}\right)^2 = \dfrac{\pi}{12}.

Let OO be the center of the larger circle. Drop the perpendicular from OO to GH\overline{GH} at J.J. Draw OG.\overline{OG}.

We have that OJG\triangle OJG is right. Since HGI=60,\angle HGI = 60^{\circ}, we also have that OJG\triangle OJG is a 30609030-60-90 triangle.

Let OJ=r.OJ = r. Then OG=2r.OG = 2r. We also have that OGOG is the sum of the height of the hexagon, equilateral triangle, and radius of the circle.

Then OG=32+3+r. OG = \dfrac{\sqrt3}{2} + \sqrt3 + r.

Substituting in OG,OG, we get 2r=32+3+r. 2r = \dfrac{\sqrt3}{2} + \sqrt3 + r. Simplifying gives us r=332. r = \dfrac{3\sqrt3}{2}.

The area of the larger circle is then π(332)2=274π. \pi \cdot \left(\dfrac{3\sqrt3}{2}\right)^2 = \dfrac{27}{4}\pi.

The desired ratio is then 27π4π12=81. \dfrac{\frac{27\pi}{4}}{\frac{\pi}{12}} = 81.

Thus, D is the correct answer.

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