2012 AIME II 第 13 题

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13.

等边三角形 ABC\triangle ABC 的边长为 111\sqrt{111}。有四个不同的三角形 AD1E1AD_1E_1AD1E2AD_1E_2AD2E3AD_2E_3AD2E4AD_2E_4,它们都与 ABC\triangle ABC 全等,并且 BD1=BD2=11BD_1 = BD_2 = \sqrt{11}。求 k=14(CEk)2\sum_{k=1}^{4}(CE_k)^2

Equilateral ABC\triangle ABC has side length 111.\sqrt{111}. There are four distinct triangles AD1E1,AD_1E_1, AD1E2,AD_1E_2, AD2E3,AD_2E_3, and AD2E4,AD_2E_4, each congruent to ABC,\triangle ABC, with BD1=BD2=11.BD_1 = BD_2 = \sqrt{11}. Find k=14(CEk)2.\sum_{k=1}^{4}(CE_k)^2.

答案:677
知识点:余弦定理变换三角恒等式
难度评级:3270
解答:

s=111s = \sqrt{111}r=11r = \sqrt{11}。因为每个三角形 ADiEkAD_iE_k 都与 ABC\triangle ABC 全等,所以 ADi=AEk=sAD_i = AE_k = s。因此 D1D_1D2D_2 是以 AA 为圆心、半径 ss 的圆与以 BB 为圆心、半径 rr 的圆的两个交点;它们关于直线 ABAB 对称,所以 BAD1=BAD2=θ\angle BAD_1 = \angle BAD_2 = \theta,且 D1D_1D2D_2 位于 ABAB 的两侧。每个 EkE_k 都是对应的 DiD_iAA 旋转 ±60\pm 60^\circ 的像。以射线 ABAB 为基准测量有向角,并令 CC 位于 +60+60^\circ,则射线 ADiAD_i 位于 ±θ\pm\theta,射线 AEkAE_k 位于 ±θ±60\pm\theta \pm 60^\circ,所以四个角 CAEk\angle CAE_kθ\thetaθ\theta120θ120^\circ - \theta120+θ120^\circ + \theta

因为 AC=AEk=sAC = AE_k = s,余弦定理给出 (CEk)2=2s2(1cosCAEk)(CE_k)^2 = 2s^2(1 - \cos\angle CAE_k)。利用 cos(120θ)\cos(120^\circ - \theta) +cos(120+θ)+ \cos(120^\circ + \theta) =2cos120cosθ= 2\cos 120^\circ \cos\theta =cosθ= -\cos\theta,四个角的余弦和为 2cosθcosθ=cosθ2\cos\theta - \cos\theta = \cos\theta,所以 k=14(CEk)2=2s2(4cosθ).\sum_{k=1}^{4}(CE_k)^2 = 2s^2(4 - \cos\theta).

在三角形 ABD1ABD_1 中应用余弦定理(其中 AB=AD1=sAB = AD_1 = s),得到 r2=2s2(1cosθ)r^2 = 2s^2(1 - \cos\theta),所以 2s2cosθ=2s2r22s^2\cos\theta = 2s^2 - r^2。因此总和等于 8s2(2s2r2)8s^2 - (2s^2 - r^2) =6s2+r2= 6s^2 + r^2 =6111+11=677= 6 \cdot 111 + 11 = 677

Write s=111s = \sqrt{111} and r=11.r = \sqrt{11}. Since each triangle ADiEkAD_iE_k is congruent to ABC,\triangle ABC, we have ADi=AEk=s,AD_i = AE_k = s, so D1D_1 and D2D_2 are the two intersections of the circle of radius ss about AA with the circle of radius rr about B;B; they are mirror images across line AB,AB, so BAD1=BAD2=θ\angle BAD_1 = \angle BAD_2 = \theta with D1D_1 and D2D_2 on opposite sides of AB.AB. Each EkE_k is the image of its DiD_i rotated ±60\pm 60^\circ about A.A. Measuring signed angles from ray AB,AB, with CC at +60,+60^\circ, the rays ADiAD_i sit at ±θ\pm\theta and the rays AEkAE_k at ±θ±60,\pm\theta \pm 60^\circ, so the four angles CAEk\angle CAE_k are θ,\theta, θ,\theta, 120θ,120^\circ - \theta, and 120+θ.120^\circ + \theta.

Since AC=AEk=s,AC = AE_k = s, the law of cosines gives (CEk)2=2s2(1cosCAEk).(CE_k)^2 = 2s^2(1 - \cos\angle CAE_k). Using cos(120θ)\cos(120^\circ - \theta) +cos(120+θ)+ \cos(120^\circ + \theta) =2cos120cosθ= 2\cos 120^\circ \cos\theta =cosθ,= -\cos\theta, the four angles' cosines sum to 2cosθcosθ=cosθ,2\cos\theta - \cos\theta = \cos\theta, so k=14(CEk)2=2s2(4cosθ).\sum_{k=1}^{4}(CE_k)^2 = 2s^2(4 - \cos\theta).

Applying the law of cosines in triangle ABD1ABD_1 (with AB=AD1=sAB = AD_1 = s) gives r2=2s2(1cosθ),r^2 = 2s^2(1 - \cos\theta), so 2s2cosθ=2s2r2.2s^2\cos\theta = 2s^2 - r^2. Therefore the sum equals 8s2(2s2r2)8s^2 - (2s^2 - r^2) =6s2+r2= 6s^2 + r^2 =6111+11=677.= 6 \cdot 111 + 11 = 677.

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