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Published byCameron Butson Modified over 2 years ago

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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

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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

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

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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

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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

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

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

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More Logical Equivalences 9

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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

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

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

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