MathDB
KMO second round #5

Source:

May 6, 2005
geometrycircumcircleincenterratiogeometric transformationgeometry unsolved

Problem Statement

A,B,CA, B, C, and DD are the four different points on the circle OO in the order. Let the centre of the scribed circle of triangle ABCABC, which is tangent to BCBC, be O1O_1. Let the centre of the scribed circle of triangle ACDACD, which is tangent to CDCD, be O2O_2.
(1) Show that the circumcentre of triangle ABO1ABO_1 is on the circle OO.
(2) Show that the circumcircle of triangle CO1O2CO_1O_2 always pass through a fixed point on the circle OO, when CC is moving along arc BDBD.