Quadrilateral

A quadrilateral is a four-sided polygon. Some special quadrilaterals include squares, rectangles, rhombi, parallelograms, trapezoids, kites, and cyclic quadrilaterals.

Introductory topics

• Cyclic quadrilaterals

Resources

Books

• Introduction to Geometry by Richard Rusczyk

Intermediate topics

• Ptolemy's Theorem
• Brahmagupta's formula

Resources

Books

• Challenging Problems in Geometry -- A good book for students who already have a solid handle on elementary geometry.
• Geometry Revisited -- A classic.