California State University, San Bernardino: Peter Williams' "Notes on Methods of Proof"

Read the following sections: "Introduction", "Definition and Theorems", "Disproving Statements", and "Types of Proofs". The types of proofs include Direct Proofs, Proof by Contradiction, Existence Proofs, and Uniqueness Proofs. You may stop the reading here; we will cover the sixth one, Mathematical Induction, later in the course.