Methods of Proof
Types of Proofs. Suppose we wish to prove an implication p → q. Here are some strategies
- Trivial Proof : If know q is true then p → q is true regardless of the truth value of p.
- Vacuous Proof : If is a conjunction of other hypotheses and we know one or more of these hypotheses is false,then p faise and so p → q is vacuously true regardless of the truth value of q.
- Direct Proof : Assume p, and then use the rules of inference,axioms,definitions,and logical equivalences to prove p.
- Indirect Proof or Proof by Contradiction : Assume p and ~q and derive a contradiction r ∧ ~r
- Proof by Contrapositive : (Special case of Proof by Contradiction.)Give a direct proof of
~q → ~p. Assume ~q and then use the rules of inference,axioms,definitions,and logical equivalences to prove ~p. (Can be thought of as a proof by contradiction in which you assume p and ~q and arrive at the contradiction p ∧ ~p.)