indirect proof (by contrapositive and by contradiction)

Watch the following videos in order:
Part 1. Axiom, Theorem, Lemma, and Corollary
Part 2. Example of trivial proof
Part 3. Example of vacuous proof
Part 4: Direct Proof
Part 5 : Direct Proof 2
Part 6. Indirect Proof
Part 7. Intro to proof by specialization and division into cases
Part 8. Example of proof by specialization and division into cases
Part 9. Example of proof by mathematical induction