MathDB
5 points concyclic wanted

Source: 2021 Dürer Math Competition Finals Day 1 E3 E+1

January 2, 2022
geometryConcyclic

Problem Statement

Let AA and BB different points of a circle kk centered at OO in such a way such that ABAB is not a diagonal of kk. Furthermore, let XX be an arbitrary inner point of the segment ABAB. Let k1k_1 be the circle that passes through the points AA and XX, and AA is the only common point of kk and k1k_1. Similarly, let k2k_2 be the circle that passes through the points BB and XX, and BB is the only common point of kk and k2k_2. Let MM be the second intersection point of k1k_1 and k2k_2. Let QQ denote the center of circumscribed circle of the triangle AOBAOB. Let O1O_1 and O2O_2 be the centers of k1k_1 and k2k_2. Show that the points M,O,O1,O2,QM,O,O_1,O_2,Q are on a circle.