Quadrilateral, Four Exterior Angle Bisectors,
Semisum, Angles. 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. A_{1}C_{1} and B_{1}D_{1} intersect at O, prove that .
Art of Geometry Problem 1243 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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