Let ABCD be a convex quadrilateral with ∠ABC>90∘,∠CDA>90∘, and ∠DAB=∠BCD. Denote by E and F the reflections of A in lines BC and CD, respectively. Suppose that the segments AE and AF meet the line BD at K and L, respectively. Prove that the circumcircles of triangles BEK and DFL are tangent to each other. (Slovakia)
Solution
Denote by A′ the reflection of A in BD. We will show that the quadrilaterals A′BKE and A′DLF are cyclic, and their circumcircles are tangent to each other at point A′. From the symmetry about line BC we have ∠BEK=∠BAK, while from the symmetry in BD we have ∠BAK=∠BA′K. Hence ∠BEK=∠BA′K, which implies that the quadrilateral A′BKE is cyclic. Similarly, the quadrilateral A′DLF is also cyclic. ! For showing that circles A′BKE and A′DLF are tangent it suffices to prove that ∠A′KB+∠A′LD=∠BA′D. Indeed, by AK⊥BC, AL⊥CD, and again the symmetry in BD we have ∠A′KB+∠A′LD=180∘−∠KA′L=180∘−∠KAL=∠BCD=∠BAD=∠BA′D, as required. Comment 1. The key to the solution above is introducing the point A′; then the angle calculations can be done in many different ways.
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