Maths Olympiad Prep

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Geometry Difficulty 6.9 National olympiad Prove it

Let ABCDA B C D be a convex quadrilateral with ABC>90,CDA>90\angle A B C>90^{\circ}, \angle C D A>90^{\circ}, and DAB=BCD\angle D A B=\angle B C D. Denote by EE and FF the reflections of AA in lines BCB C and CDC D, respectively. Suppose that the segments AEA E and AFA F meet the line BDB D at KK and LL, respectively. Prove that the circumcircles of triangles BEKB E K and DFLD F L are tangent to each other. (Slovakia)

Solution

Denote by AA^{\prime} the reflection of AA in BDB D. We will show that the quadrilaterals ABKEA^{\prime} B K E and ADLFA^{\prime} D L F are cyclic, and their circumcircles are tangent to each other at point AA^{\prime}. From the symmetry about line BCB C we have BEK=BAK\angle B E K=\angle B A K, while from the symmetry in BDB D we have BAK=BAK\angle B A K=\angle B A^{\prime} K. Hence BEK=BAK\angle B E K=\angle B A^{\prime} K, which implies that the quadrilateral ABKEA^{\prime} B K E is cyclic. Similarly, the quadrilateral ADLFA^{\prime} D L F is also cyclic. ! For showing that circles ABKEA^{\prime} B K E and ADLFA^{\prime} D L F are tangent it suffices to prove that
AKB+ALD=BAD. \angle A^{\prime} K B+\angle A^{\prime} L D=\angle B A^{\prime} D.
Indeed, by AKBCA K \perp B C, ALCDA L \perp C D, and again the symmetry in BDB D we have
AKB+ALD=180KAL=180KAL=BCD=BAD=BAD, \angle A^{\prime} K B+\angle A^{\prime} L D=180^{\circ}-\angle K A^{\prime} L=180^{\circ}-\angle K A L=\angle B C D=\angle B A D=\angle B A^{\prime} D,
as required. Comment 1. The key to the solution above is introducing the point AA^{\prime}; then the angle calculations can be done in many different ways.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.