Compound Statements

Unit 9 > Lesson 5 of 11

Now that we have learned about negation, conjunction, disjunction and the conditional, we can include the logical connector for each of these statements in more elaborate statements. In this lesson, we will learn how to determine the truth values of a compound statement with the logical connectors ~, , and .

Example 1:
Given: p: 72 = 49 true q: A rectangle does not have 4 sides. false r: Harrison Ford is an American actor. true Problem: Write each sentence below in symbolic form. Then determine its truth value.

1.	If 72 = 49, then a rectangle has 4 sides.	p~q	true
2.	If 72 49, then a rectangle does not have 4 sides.	~pq	true
3.	If a rectangle has 4 sides, then Harrison Ford is not an American actor.	~q~r	false
4.	If Harrison Ford is an American actor, then 72 49.	r~p	false
5.	If 72 = 49 or a rectangle does not have 4 sides, then Harrison Ford is not an American actor.	(pq)~r	false

In Example 1, each of the first four sentences is represented by a conditional statement in symbolic form. In item 5, (pq)~r is a compound statement that includes the connectors , , and ~. It is easier to determine the truth value of such an elaborate compound statement when a truth table is constructed as shown below.

p	q	~r	pq	(pq)~r
T	F	F	T	F

Example 2:

Given:	p: 28 is a multiple of 7.	true
	q: 7 is an even number.	false
Problem:	Determine the truth value of this compound statement: ~p(qp)

Solution:

p	q	~p	qp	~p (qp)
T	F	F	T	T

Example 3:

Given:	a is true, b is false, and c is true.
Problem:	Determine the truth value of this compound statement: (~ac)b

Solution:

a	b	c	~a	~ac	(~ac)b
T	F	T	F	F	T

Example 4:  What are the truth values of this compound statement? (pq)q

p	q	pq	(pq)q
T	T	T	T
T	F	T	F
F	T	T	T
F	F	F	T

Solution: The truth values of (pq)q are {T, F, T, T}.

Example 5:  What are the truth values of this compound statement?   q (p~q)

p	q	~q	p~q	q (p~q)
T	T	F	F	F
T	F	T	T	T
F	T	F	F	F
F	F	T	F	T

Solution: The truth values of q (p~q) are {F, T, F, T}.

Example 6:  What are the truth values of this compound statement?   (sr)~r

r	s	~r	sr	(sr)~r
T	T	F	T	F
T	F	F	T	F
F	T	T	F	F
F	F	T	T	T

Solution: The truth values of (sr)~r are {F, F, F, T}.

Example 7:  What are the truth values of this compound statement?   ~b(ab)

a	b	~b	ab	~b(ab)
T	T	F	T	T
T	F	T	T	T
F	T	F	T	T
F	F	T	F	F

Solution: The truth values of ~b(ab) are {T, T, T, F}.

Summary:

We have learned how to write a sentence as a compound statement in symbolic form. We have learned how to determine the truth values of a compound statement with the logical connectors ~, , and .

Exercises

1.	Given: a: 11 is prime. b: 11 is odd. Problem: Which of the following sentences represents (ab)~b?
	If 11 is prime and 11 is odd, then 11 is not odd. If 11 is prime or 11 is not odd, then 11 is not odd. If 11 is prime or 11 is odd, then 11 is not odd. None of the above. RESULTS BOX:

2.	If r and s are false statements, then what is the truth value of (~rs)s?
	True False Not enough information was given. None of the above. RESULTS BOX:

3.	If x and y are true statements, then what is the truth value of (xy)~y?
	True False Not enough information was given. None of the above. RESULTS BOX:

4.	What are the truth values of this statement?  (~xy)y
	{T, T, T, F} {T, T, T, T} {T, F, T, T} None of the above. RESULTS BOX:

5.	What are the truth values of this statement?  ~p(p~q)
	{T, F, T, F} {F, T, F, T} {T, T, F, F} None of the above. RESULTS BOX:

Lessons on Symbolic Logic Negation Conjunction Disjunction Conditional Compound Biconditional Tautologies Equivalence Practice Exercises Challenge Exercises Solutions Related Activities Interactive Puzzle Printable Worksheets