2017 AMC 12B 第 24 题

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24.

四边形 ABCDABCD 在 BB 和 CC 处有直角,△ABC∼△BCD\triangle ABC \sim \triangle BCD,且 AB>BCAB \gt BC。在 ABCDABCD 内部有一点 EE,使得 △ABC∼△CEB\triangle ABC \sim \triangle CEB,并且 △AED\triangle AED 的面积是 △CEB\triangle CEB 面积的 1717 倍。ABBC\dfrac{AB}{BC} 是多少?

Quadrilateral ABCDABCD has right angles at BB and C,C, △ABC∼△BCD,\triangle ABC \sim \triangle BCD, and AB>BC.AB \gt BC. There is a point EE in the interior of ABCDABCD such that △ABC∼△CEB\triangle ABC \sim \triangle CEB and the area of △AED\triangle AED is 1717 times the area of △CEB.\triangle CEB. What is ABBC?\dfrac{AB}{BC}?

1+21 + \sqrt{2}

2+22 + \sqrt{2}

17\sqrt{17}

2+52 + \sqrt{5}

1+231 + 2\sqrt{3}

答案:D
知识点:相似坐标几何鞋带公式
难度评级:2550
小提示:

令 BC=1BC = 1,AB=rAB = r,并取 C=(0,0)C = (0,0),B=(0,1)B = (0,1),A=(r,1)A = (r,1),D=(1r,0)D = (\tfrac1r, 0)

Set BC=1,BC = 1, AB=r,AB = r, and place C=(0,0),C = (0,0), B=(0,1),B = (0,1), A=(r,1),A = (r,1), D=(1r,0)D = (\tfrac1r, 0)

大提示:

由 △ABC∼△CEB\triangle ABC \sim \triangle CEB,求出 EE,计算两个面积,并令 [△AED]=17[△CEB][\triangle AED] = 17[\triangle CEB]。

Find EE from △ABC∼△CEB,\triangle ABC \sim \triangle CEB, compute both areas, and set [△AED]=17[△CEB][\triangle AED] = 17[\triangle CEB]

解答:

令 BC=1BC = 1,AB=r>1AB = r \gt 1。由 △ABC∼△BCD\triangle ABC \sim \triangle BCD 以及直角位置,可取 C=(0,0)C = (0,0),B=(0,1)B = (0,1),A=(r,1)A = (r,1),D=(1r,0)D = \bigl(\tfrac1r, 0\bigr)。设 E=(x,y)E = (x, y),其中 x,y>0x, y \gt 0。由 △ABC∼△CEB\triangle ABC \sim \triangle CEB 得 xy=tan⁡(∠ECB)\dfrac{x}{y} = \tan(\angle ECB) =tan⁡(∠BAC)= \tan(\angle BAC) =1r= \dfrac1r 且 x2+y2=r21+r2x^2 + y^2 = \dfrac{r^2}{1 + r^2},所以 x=r1+r2x = \dfrac{r}{1+r^2},y=r21+r2y = \dfrac{r^2}{1+r^2}。这两个相关的面积为 [△CEB]=r2(1+r2),[△AED]=r4−r2+12r(1+r2)。 \begin{aligned} [\triangle CEB]&=\dfrac{r}{2(1+r^2)},\\ [\triangle AED]&=\dfrac{r^4-r^2+1}{2r(1+r^2)} \end{aligned}\text{。} 令后者等于前者的 1717 倍,得到 r4−18r2+1=0r^4 - 18r^2 + 1 = 0。于是 r2=9+45=(2+5)2r^2 = 9 + 4\sqrt5 = (2 + \sqrt5)^2,所以 r=2+5r = 2 + \sqrt5。

所以正确答案是 D。

Set BC=1BC = 1 and AB=r>1.AB = r \gt 1. The similarity △ABC∼△BCD\triangle ABC \sim \triangle BCD with the right angles places the figure at C=(0,0),C = (0,0), B=(0,1),B = (0,1), A=(r,1),A = (r,1), D=(1r,0).D = \bigl(\tfrac1r, 0\bigr). Let E=(x,y)E = (x, y) with x,y>0.x, y \gt 0. From △ABC∼△CEB\triangle ABC \sim \triangle CEB we get xy=tan⁡(∠ECB)\dfrac{x}{y} = \tan(\angle ECB) =tan⁡(∠BAC)= \tan(\angle BAC) =1r= \dfrac1r and x2+y2=r21+r2,x^2 + y^2 = \dfrac{r^2}{1 + r^2}, so x=r1+r2,x = \dfrac{r}{1+r^2}, y=r21+r2.y = \dfrac{r^2}{1+r^2}. The two relevant areas are [△CEB]=r2(1+r2),[△AED]=r4−r2+12r(1+r2). \begin{aligned} [\triangle CEB]&=\dfrac{r}{2(1+r^2)},\\ [\triangle AED]&=\dfrac{r^4-r^2+1}{2r(1+r^2)}. \end{aligned} Setting the second equal to 1717 times the first gives r4−18r2+1=0.r^4 - 18r^2 + 1 = 0. Then r2=9+45=(2+5)2,r^2 = 9 + 4\sqrt5 = (2 + \sqrt5)^2, so r=2+5.r = 2 + \sqrt5.

Thus, the correct answer is D.

第 23 题#23
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