The Open Door Web Site

 

IB MATH STUDIES

SETS, LOGIC AND PROBABILITY

LOGIC

A proposition 

In logic a proposition is a statement that has the answer true (T) or false (F)

Examples:

Today is Friday

  

My name is Fred

  

It is sunny

  
   
What day is it? This is not a proposition.

Propositions are represented by the letters p, q , r in the IB exams

Example:

p Today is Friday
q It is sunny
r I will go for a walk
 

** The above three propositions will be used in this section on logic

Logical operators

Joining propositions together with logical operators forms compound propositions.

1. Negation The word ‘not’ or ‘do not’ etc. is put in front of the original proposition.
  Example:    
    p Today is not Friday
       
  NB.  Negation is not the opposite of the original proposition i.e. "if the hotel is full" is the first proposition, then the negation is its "not full".  We can't say "empty", which would be the opposite.
 

Given the proposition 
   

q

 "the hotel is full"
 

then

q  is "the hotel is not full"
 

(we can’t say its empty, as it may have 1 room occupied or 2 etc)

Truth tables

For each compound proposition we can calculate its truth value using a Truth Table.

p p
T F
F T

If the original proposition is True then the negation of it must be False.

2. Conjunction The word AND is used to join two propositions together.
   

p q

Today is Friday and it is sunny.
 
p q p q
T T T
T F F
F T F
F F F
 
 
 

With conjunction the overall proposition is only True when both of the propositions are true.
     
3. Disjunction The word OR is used to join the two propositions together
    p q Today is Friday or it is sunny  (or both)
 
p q p q
T T T
T F T
F T T
F F F
 
 
 
 

Only one of the propositions has to be True to make the compound proposition true.
       
4. Exclusive disjunction

The word OR is again used to join the two propositions together, but this time it is not both.
   

p q

 Today is Friday or it is sunny , but not both.
 
p q p q
T T F
T F T
F T T
F F F
 
 
 

Only one of the original propositions must be true for the compound proposition to be true.
       
5. Implication

The words If …. Then …….. are used to join the two propositions together.

(The first proposition implies the second) Often thought of as the first proposition being the hypothesis and the second the conclusion.
   

p q

 If it is Friday then it is sunny
 
p q p q
T T T
T F F
F T T
F F T
 
 
 

The compound proposition is only false when the first proposition is True and the second False.
       
6. Equivalence The words If and only if are used to join the two propositions.
   

p q

  It is Friday if and only if it is sunny.
 
p q p q
T T T
T F F
F T F
F F T
 
 
 

The compound proposition is True if the truth values of the two propositions are the same (equivalent)

       
7.

Converse, Inverse and Contrapositive
   

Given p q :  If it is Friday then it is sunny, is the implication
   

Then the Converse is q p : If it is sunny then it is Friday.
   

The Inverse is p q : If it is not Friday then it is not sunny.
   

The contrapositive is q p : If it is not sunny then it is not Friday.
       
 

Two pairs of the above statements are logically equivalent (have the same truth values as can be seen from the truth table below).
 
p q p q p q q p p q q p
T T F F T T T T
T F F T F T T F
F T T F T F F T
F F T T T T T T
 

Implication and the contrapositive are logically equivalent as are the Converse and inverse.
       
8.

Tautology and Contradiction
 

A tautology is a compound proposition (statement) that always has the truth value True, whatever the truth values of the individual propositions. A contradiction always has the truth value False.

  Example  (pq) (pq)  is a contradiction as can be seen from the truth table below.
 
p q q pq pq (pq) (pq)
T T F F T F
T F T T F F
F T F F T F
F F T F T F
       
  Logic and Truth Table Summary    
  Questions    
  Top of Page    

Privacy Policy

Copyright Information

Sponsored Links

Sponsored Pages

Donating to the ODWS

Advertising on the ODWS