2025 AIME II Problem 14

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Concepts:equilateral triangletrigonometric identityarea decomposition

Difficulty rating: 3270

14.

Let ABC\triangle ABC be a right triangle with A=90\angle A = 90^\circ and BC=38.BC = 38. There exist points KK and LL inside the triangle such that AK=AL=BK=CL=KL=14.AK = AL = BK = CL = KL = 14. The area of the quadrilateral BKLCBKLC can be expressed as n3n\sqrt{3} for some positive integer n.n. Find n.n.

Solution:

Since AK=AL=KL=14,AK = AL = KL = 14, triangle AKLAKL is equilateral and KAL=60.\angle KAL = 60^\circ. Let α=BAK\alpha = \angle BAK and β=LAC,\beta = \angle LAC, so α+β=30.\alpha + \beta = 30^\circ. Because AK=KB,AK = KB, point KK lies on the perpendicular bisector of AB,\overline{AB}, so AB=214cosα=28cosα;AB = 2 \cdot 14\cos\alpha = 28\cos\alpha; similarly AC=28cosβ.AC = 28\cos\beta. Then AB2+AC2=382AB^2 + AC^2 = 38^2 gives cos2α+cos2β=361196,\cos^2\alpha + \cos^2\beta = \frac{361}{196}, i.e. cos2α+cos2β=16598.\cos 2\alpha + \cos 2\beta = \frac{165}{98}. By sum-to-product, 2cos(α+β)cos(αβ)=3cos(αβ)=16598,2\cos(\alpha+\beta)\cos(\alpha-\beta) = \sqrt{3}\cos(\alpha - \beta) = \frac{165}{98}, so cos(αβ)=55398.\cos(\alpha - \beta) = \frac{55\sqrt{3}}{98}.

Decompose [BKLC]=[ABC][ABK][ACL][AKL].[BKLC] = [ABC] - [ABK] - [ACL] - [AKL]. First, [ABC]=12ABAC=392cosαcosβ=196(cos(αβ)+cos(α+β))=1103+983=2083.[ABC] = \tfrac{1}{2} AB \cdot AC = 392\cos\alpha\cos\beta = 196\bigl(\cos(\alpha - \beta) + \cos(\alpha + \beta)\bigr) = 110\sqrt{3} + 98\sqrt{3} = 208\sqrt{3}. Next, KK has height 14sinα14\sin\alpha over AB,\overline{AB}, so [ABK]=1228cosα14sinα=98sin2α,[ABK] = \frac{1}{2} \cdot 28\cos\alpha \cdot 14\sin\alpha = 98\sin 2\alpha, and likewise [ACL]=98sin2β;[ACL] = 98\sin 2\beta; their sum is 196sin(α+β)cos(αβ)=9855398=553.196\sin(\alpha + \beta)\cos(\alpha - \beta) = 98 \cdot \frac{55\sqrt{3}}{98} = 55\sqrt{3}. Finally [AKL]=34142=493.[AKL] = \frac{\sqrt{3}}{4} \cdot 14^2 = 49\sqrt{3}.

Therefore [BKLC]=2083553493=1043,[BKLC] = 208\sqrt{3} - 55\sqrt{3} - 49\sqrt{3} = 104\sqrt{3}, so n=104.n = 104.

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