This Oswego City School District Regents Exam Prep site shows you how to proof something by showing that it is false. This look at indirect proofs includes a practice and a teacher resource page, which features a warm-up activity.

This site from Cut The Knot provides an overview of the law of cosines, complete with an explanation of the law and a proof of the law.

An explanation and proof of the Four Color Map Theorem. Includes illustrative images and a plethora of information.