Quadrilateral

A quadrilateral is a four-sided geometric figure. Some common quadrilaterals are squares, rectangles, rombuses, parrallelograms, and trapezoids.

Introductory topics

• cyclic quadrilaterals

Resources

Books

• Introduction to Geometry by Richard Rusczyk

Intermediate topics

• Ptolemy's Theorem

Resources

Books

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