### Proposition

Let a cyclic quadrilateral ABCD. Then the sum of the products of the two pairs of opposite sides equals the product of its two diagonals. In other words the rectangle contained by the diagonals of any cyclic quadrilateral ABCD is equal to the sum of the rectangles contained by the pairs of opposite sides.

## Sketch of Ptolemy's Theorem using iPad Apps

## Geometric Art
using Mobile Apps

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.