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# Chapter 1: The Foundations: Logic and Proofs - PowerPoint PPT Presentation

Chapter 1: The Foundations: Logic and Proofs. 1.1 Propositional Logic 1.2 Propositional Equivalences 1.3 Predicates and Quantifiers 1.4 Nested Quantifiers 1.5 Rules of Inference 1.6 Introduction to Proofs 1.7 Proof Methods and Strategy. 1.2: Propositional Equivalences. Definition:

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### Chapter 1: The Foundations: Logic and Proofs

1.1 Propositional Logic

1.2 Propositional Equivalences

1.3 Predicates and Quantifiers

1.4 Nested Quantifiers

1.5 Rules of Inference

1.6 Introduction to Proofs

1.7 Proof Methods and Strategy

### 1.2: Propositional Equivalences

Definition:

Tautology: A compound proposition that is always true.

Contradiction: A compound proposition that is always false.

Contingency: A compound proposition that is neither a tautology nor a contradiction.

### Logical Equivalences

Compound propositions that have the same truth values in all possible cases are called logically equivalent.

Definition:

The compound propositions p and q are called logically equivalent if pq is a tautology. Denote pq.

### Logical Equivalences

One way to determine whether two compound propositions are equivalent is to use a truth table.

Symbol: PQ

• Prove the De Morgan’s Laws.

• HW: Prove the other one (De Morgan’s Laws).

Example:

Show that pq and ¬pqare logically equivalent.

HW: example 4 of page 23

t01_2_006.jpg

Example 5: Use De Morgan’s laws to express the negations of “Miguel has a cellphone and he has a laptop computer”.

Example 5: Use De Morgan’s laws to express the negations of “Heather will go to the concert or Steve will go to the concert”.

• Example 6: Show that ¬(pq) and p ¬q are logically equivalent.

• Example 7: Show that ¬(p(¬p  q)) and ¬p  ¬q are logically equivalent by developing a serious of logical equivalences.

• Example 8: Show that (p  q) (pq) is a tautology.

• Tautology