Online Geometry Problem 610: Intersecting Circles, Radius, Parallel, Common Tangent, Angle, 90 Degrees. Level: Mathematics Education, High School, College

The figure shows the intersecting circles A and B at points C and D. A radius AE is parallel to radius BF. Lines AB and EF meet at G. The common external tangent HM and AB extended meet at N. Prove that the measure of angle GCN is 90 degrees.