Presentation is loading. Please wait.

Presentation is loading. Please wait.

1. Tautologies, Contradictions, and Contingencies A tautology is a proposition that is always true. Example: p ¬p A contradiction is a proposition that.

Similar presentations


Presentation on theme: "1. Tautologies, Contradictions, and Contingencies A tautology is a proposition that is always true. Example: p ¬p A contradiction is a proposition that."— Presentation transcript:

1 1

2 Tautologies, Contradictions, and Contingencies A tautology is a proposition that is always true. Example: p ¬p A contradiction is a proposition that is always false. Example: p ¬p A contingency is a compound proposition that is neither a tautology nor a contradiction P¬p¬pp ¬pp ¬pp ¬pp ¬p TFTF FTTF 2

3 Equivalent Propositions Two propositions are equivalent if they always have the same truth value. Formally: Two compound propositions p and q are logically equivalent if pq is a tautology. We write this as pq ( or pq) One way to determine equivalence is to use truth tables Example: show that ¬p q is equivalent to p q. 3

4 Equivalent Propositions Example: Show using truth tables that that implication is equivalent to its contrapositive Solution: 4

5 Show Non-Equivalence Example: Show using truth tables that neither the converse nor inverse of an implication are equivalent to the implication. Solution: pq¬ p¬ qp q¬ p ¬ qq p TTFFTTT TFFTFTT FTTFTFF FFTTTTT 5

6 De Morgans Laws pq¬p¬p¬q¬q ( pq)¬ ( pq)¬p¬q¬p¬q TTFFTFF TFFTTFF FTTFTFF FFTTFTT Augustus De Morgan Very useful in constructing proofs This truth table shows that De Morgans Second Law holds

7 Key Logical Equivalences Identity Laws:, Domination Laws:, Idempotent laws:, Double Negation Law: Negation Laws:,

8 Key Logical Equivalences (cont) Commutative Laws:, Associative Laws: Distributive Laws: Absorption Laws:

9 More Logical Equivalences 9

10 Equivalence Proofs Instead of using truth tables, we can show equivalence by developing a series of logically equivalent statements. To prove that A B we produce a series of equivalences leading from A to B. Each step follows one of the established equivalences (laws) Each A i can be an arbitrarily complex compound proposition. 10

11 Equivalence Proofs Example: Show that is logically equivalent to Solution: 11 by the negation law

12 Equivalence Proofs Example: Show that is a tautology. Solution: 12 by equivalence from Table 7 by the negation law (¬q q)


Download ppt "1. Tautologies, Contradictions, and Contingencies A tautology is a proposition that is always true. Example: p ¬p A contradiction is a proposition that."

Similar presentations


Ads by Google