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the centers of the circles are on opposite sides of AB

Source: 6th JBMO 2002, Problem 2

June 19, 2004
geometrytrapezoid

Problem Statement

Two circles with centers O1O_{1} and O2O_{2} meet at two points AA and BB such that the centers of the circles are on opposite sides of the line ABAB. The lines BO1BO_{1} and BO2BO_{2} meet their respective circles again at B1B_{1} and B2B_{2}. Let MM be the midpoint of B1B2B_{1}B_{2}. Let M1M_{1}, M2M_{2} be points on the circles of centers O1O_{1} and O2O_{2} respectively, such that AO1M1=AO2M2\angle AO_{1}M_{1}= \angle AO_{2}M_{2}, and B1B_{1} lies on the minor arc AM1AM_{1} while BB lies on the minor arc AM2AM_{2}. Show that MM1B=MM2B\angle MM_{1}B = \angle MM_{2}B. Ciprus