2010 AMC 10B 第 20 题
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20.
两个圆位于正六边形 外。第一个圆与 相切,第二个圆与 相切。两个圆都与直线 和 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 The first is tangent to and the second is tangent to Both are tangent to lines and What is the ratio of the area of the second circle to that of the first circle?
答案:D
解答:
参考下图:
设正六边形边长为 。较小圆内切于一个边长为 的等边三角形。
这个等边三角形的内切圆半径为 。面积为
设较大圆圆心为 ,从 向 作垂线,垂足为 。并连接 。
是直角三角形,且由 可知, 是 三角形。
设 。由三十、六十、九十度三角形关系,。接下来用 表示较大圆的位置。
于是
代入 ,得 化简得
较大圆面积为
所求比值为
所以正确答案是 D。
Consider the following diagram:
Assume the regular hexagon has side length The smaller circle is inscribed in an equilateral triangle of side length
The inradius of this equilateral triangle is The area of the circle is then
Let be the center of the larger circle. Drop the perpendicular from to at Draw
We have that is right. Since we also have that is a triangle.
Let Then We also have that is the sum of the height of the hexagon, equilateral triangle, and radius of the circle.
Then
Substituting in we get Simplifying gives us
The area of the larger circle is then
The desired ratio is then
Thus, D is the correct answer.
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