parallelogram criterion related to projections on 2 concentric circles
Source: Sharygin 2010 Final 10.4
November 25, 2018
geometryparallelogramconcentric circlesconcentricprojectionsinscribed
Problem Statement
Projections of two points to the sidelines of a quadrilateral lie on two concentric circles (projections of each point form a cyclic quadrilateral and the radii of circles are different). Prove that this quadrilateral is a parallelogram.