olymmath

ISL 2020 G5

Let ABCDA B C D be a cyclic quadrilateral with no two sides parallel. Let K,L,MK, L, M, and NN be points lying on sides AB,BC,CDA B, B C, C D, and DAD A, respectively, such that KLMNK L M N is a rhombus with KL∥ACK L \| A C and LM∥BDL M \| B D. Let ω1,ω2,ω3\omega_{1}, \omega_{2}, \omega_{3}, and ω4\omega_{4} be the incircles of triangles ANKA N K, BKL,CLMB K L, C L M, and DMND M N, respectively. Prove that the internal common tangents to ω1\omega_{1} and ω3\omega_{3} and the internal common tangents to ω2\omega_{2} and ω4\omega_{4} are concurrent. (Poland)

Official text Transcription from NuminaMath olympiads, checked word for word against the official paper. Quoted with source.

Official text: IMO 2020 Shortlist booklet, page 10.

Solutions

Official solution Solution 1 by the IMO Problem Selection Committee

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