2006 AIME I 第 7 题

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7.

如图,在一组等距平行线上画出一个角。阴影区域 C\mathcal{C} 的面积与阴影区域 B\mathcal{B} 的面积之比为 115\frac{11}{5}。求阴影区域 D\mathcal{D} 的面积与阴影区域 A\mathcal{A} 的面积之比。

An angle is drawn on a set of equally spaced parallel lines as shown. The ratio of the area of shaded region C\mathcal{C} to the area of shaded region B\mathcal{B} is 115.\frac{11}{5}. Find the ratio of the area of shaded region D\mathcal{D} to the area of shaded region A.\mathcal{A}.

答案:408
知识点:相似面积比平方差
难度评级:2500
小提示:

令平行线间距为一,每个区域都是相似三角形面积之差,所以面积随到顶点距离的平方缩放

With unit spacing, every region is a difference of similar triangles, so areas scale as squares of distances from the vertex

大提示:

xx 是顶点到第一条线的距离,则 CB=2x+72x+3=115\frac{\mathcal{C}}{\mathcal{B}} = \frac{2x+7}{2x+3} = \frac{11}{5} 可确定 xx

If xx is the vertex’s distance to the first line, then CB=2x+72x+3=115\frac{\mathcal{C}}{\mathcal{B}} = \frac{2x+7}{2x+3} = \frac{11}{5} determines xx

解答:

取相邻平行线的间距为单位长度,令 xx 为角的顶点到第一条线的距离。第 jj 条线截出的三角形与 A\mathcal{A} 相似,相似比为 x+j1x\frac{x + j - 1}{x},所以其面积与 (x+j1)2(x + j - 1)^2 成正比,而第 jj 条线与第 j+1j + 1 条线之间的条带面积与 (x+j)2(x + j)^2 (x+j1)2=2x+2j1- (x + j - 1)^2 = 2x + 2j - 1 成正比。

区域 B\mathcal{B}C\mathcal{C}D\mathcal{D} 分别是从第 224466 条线开始的条带,面积分别与 2x+32x + 32x+72x + 72x+112x + 11 成正比。已知比值得 2x+72x+3=115\frac{2x+7}{2x+3} = \frac{11}{5},所以 10x+35=22x+3310x + 35 = 22x + 33,从而 x=16x = \frac{1}{6}

因为 A\mathcal{A} 的面积与 x2x^2 成正比,所求比值为 2x+11x2=343136=408\frac{2x + 11}{x^2} = \frac{\frac{34}{3}}{\frac{1}{36}} = 408

Take the spacing between consecutive lines as the unit, and let xx be the distance from the vertex of the angle to the first line. The triangle cut off by the jjth line is similar to triangle A\mathcal{A} with ratio x+j1x,\frac{x + j - 1}{x}, so its area is proportional to (x+j1)2,(x + j - 1)^2, and the strip between lines jj and j+1j + 1 has area proportional to (x+j)2(x + j)^2 (x+j1)2=2x+2j1.- (x + j - 1)^2 = 2x + 2j - 1.

Regions B,\mathcal{B}, C,\mathcal{C}, and D\mathcal{D} are the strips beginning at lines 2,2, 4,4, and 6,6, with areas proportional to 2x+3,2x + 3, 2x+7,2x + 7, and 2x+11.2x + 11. The given ratio yields 2x+72x+3=115,\frac{2x+7}{2x+3} = \frac{11}{5}, so 10x+35=22x+3310x + 35 = 22x + 33 and x=16.x = \frac{1}{6}.

Since A\mathcal{A} has area proportional to x2,x^2, the requested ratio is 2x+11x2=343136=408.\frac{2x + 11}{x^2} = \frac{\frac{34}{3}}{\frac{1}{36}} = 408.

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