Home Search Geometry Problems Collinearity Concurrency 1241-1250 Open Problems Visual index Post or view a solution
 Quadrilateral, Four Interior and Exterior Angle Bisectors, Collinearity, Concurrent Lines. Mobile Apps In a quadrilateral ABCD, the bisectors of the exterior angles A, B, C, and D intersect at A1, B1, C1, and D1, as shown in the figure below. The bisectors of the interior angles A, B, C, and D intersect at A2, B2, C2, and D2. Prove that (1) A1, A2, C2, and C1 are collinear, similarly B1, D2, B2, and D1 are collinear; (2) Lines AD, BC, and A1C1 extended are concurrent, similarly AB, DC, and D1B1 extended are concurrent. Art of Geometry Problem 1242 using Mobile Apps. Circle Patterns. Geometric art is a form of art based on the use and application of geometric figures. A geometric figure is any set or combination of points, lines, surfaces and solids. A mobile app or mobile application software is a computer program designed to run on smartphones and tablet computers. Recent Additions