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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.

 Geometry problem 1242 Quadrilateral, Four Interior and Exterior Angle Bisectors, Collinearity, Concurrent Lines
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.

Art of problem 1242 using mobile apps, iPad, iPhone
 

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