Direct, contrapositive, induction, contradiction, counting, and inequalities.
Formalizes foundational deductive proof paradigms: (1) Direct Proof: assumes premise \(P\) and applies definitions/axioms to derive \(Q\); (2) Proof by Contraposition: establishes \(\neg Q \implies \neg P\), logically equivalent to \(P \implies Q\); (3) Proof by Contradiction: assumes \(P \land \neg Q\) and derives a logical absurdity \(\bot\) (e.g., Hippasus proof that \(\sqrt{2} \notin \mathbb{Q}\)); (4) Mathematical Induction: proves base case \(P(0)\) and inductive step \(\forall k, P(k) \implies P(k+1)\).
Assume P is true, then through a logical sequence of steps, show Q must also be true.
"P ⟹ Q" is logically equivalent to "¬Q ⟹ ¬P". Sometimes proving the contrapositive is easier.
Assume ¬P is true, then derive a contradiction. Therefore P must be true.