University of Saskatchewan (Canada)
University of Saskatchewan: Techniques for Proofs
The University of Saskatchewan's Department of Mathematics and Statistics provides high school students with online tutorials and exercises on what they deem some of the most important techniques for mathematical proofs: truth tables,...
Oswego City School District
Regents Exam Prep Center: Indirect Euclidean Proofs
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.