In a triangle ABC, the excircle or escribed circle of center O relative to AC is tangent to the sides at D, E, and F, as shown in the figure. CO cuts DE at C1. Prove that angle AC1C = 90 degrees.
Geometry Problems Visual Index 1261-1270 Open Problems All Problems Triangles Angle Bisector Circle Excircle Tangent Line 90 Degrees View or Post a solution