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// lines connecting 2 centers, # - All-Russian MO 2001 Regional (R4) 10.2

Source:

September 26, 2024
geometryparallelparallelogram

Problem Statement

In parallelogram ABCDABCD, point KK is marked on diagonal ACAC. Circle s1s_1 passes through point KK and touches lines ABAB and ADAD (s1s_1 intersects the diagonal ACAC for the second time on the segment AKAK). Circle s2s_2 passes through point KK and touches lines CBCB and CDCD (s2s_2 intersects for the second time diagonal ACAC on segment KCKC). Prove that for all positions of the point KK on the diagonal ACAC, the straight lines connecting the centers of circles s1 s_1 and s2s_2, will be parallel to each other.