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 A_{1}, B_{1}, C_{1}, and D_{1}, as shown in the figure below. The bisectors of the interior angles A, B, C, and D intersect at A_{2}, B_{2}, C_{2}, and D_{2}. Prove that (1) A_{1}, A_{2}, C_{2}, and C_{1} are collinear, similarly B_{1}, D_{2}, B_{2}, and D_{1} are collinear; (2) Lines AD, BC, and A_{1}C_{1} extended are concurrent, similarly AB, DC, and D_{1}B_{1} 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.

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