2012 AIME II Problem 13

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13.

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.

Answer: 677
Concepts:law of cosinestransformationtrigonometric identity
Difficulty rating: 3270
Solution:

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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