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Section 1.4 Supplemental Exercises
Subsection 1.4.1 Logic Questions
Exercises Exercises
1.
Which of the following are statements?
It rained yesterday.
All cars have engines.
Did you see the game last night?
All dogs go to Heaven.
This statement is false.
Come visit sometime.
2.
Joe says "Dolphins are mammals or live primarily underwater." Is his statement true?
3.
Given each truth table below, provide a statement it represents (that is, define P and Q and determine the relationship between them).
P
Q
T
T
T
F
T
F
T
F
F
F
F
F
P
Q
T
T
T
F
T
T
T
F
T
F
F
F
P
Q
T
T
F
F
T
F
T
F
F
F
F
T
P
Q
T
T
T
F
T
T
T
F
T
F
F
T
4.
Which of the following are open sentences? For each open sentence, identify its domain.
\(\displaystyle x+3<5\)
Dog \(D\) is a mastiff.
Vitamin C helps prevent scurvy.
The statement \(P\) is both true and false.
Some countries have kings.
No one lives in a house.
5.
For each open sentence in the previous question, give a value of the variable that makes the statement true.
6.
Layla says "Statement
\(P\) is false." How does the truth value of this open sentence depend on the truth value of the variable
\(P\text{?}\)
7.
Draw a truth table for each of the following.
The cow is awake and the pig is asleep.
Stores accept cash or credit as payment.
Education prepares you for a career or is a poor way to spend your time.
Anna and Jordan are not happy.
8.
Write a negation of each of the following.
Gravity is not a force.
City \(C\) has a population of at least 10 million.
Harold ate a burger and fries for lunch.
There is at least one penguin that has never seen Antarctica.
Everyone on campus is a student or a staff-member.
No beverage is hot and sweet.
9.
For each of the following statements, give a logically equivalent statement with the same domain.
Peter is not unhappy.
Not all books are informative or interesting.
All sculptures are not made of marble.
No film has sound.
\(\displaystyle n-2=9\)
Some buses have 4 tires.
10.
Draw (and label) a Venn diagram to illustrate each of the following quantified statements.
All parrots are birds.
Some widgets are gizmos.
No mountains are volcanoes.
Songs are not all classical.
At least one article has no sources.
Not all athletes have strict diets and exercise every day.
Some vegans eat honey, but no vegans eat chicken.
All mammals breathe oxygen, but not all oxygen-breathers are mammals.
11.
Please note: a compound statement made up of three statements (P, Q, R) has a truth table of the following form:
P
Q
R
T
T
T
F
T
T
T
F
T
F
F
T
T
T
F
F
T
F
T
F
F
F
F
F
How many rows would be required to construct a truth table for "P and Q or R or S"?
Construct a truth table for "(P or Q) and R."
Is "(P or R) and Q" logically equivalent to "(P or Q) and R"? Why or why not?
Subsection 1.4.2 Conditional Questions
Exercises Exercises
1.
Which of the following are conditional statements?
When it rains, it pours.
If you have any snacks left over, can I have them?
If the stock market crashes, the economy will enter a recession.
\(c+d\) is even if \(c\) and \(d\) are both multiples of 3.
Let someone else hold your phone if you’re scared of dropping it.
If you want to cross the street, then make sure to look both ways for traffic first.
2.
Is the following a valid direct proof of “if
\(n\) is even then
\(n^3\) is even”? How do you know?
Assume
\(n=2\text{.}\) Then,
\(n^3=8\text{.}\) Since 2 and 8 are even, we have shown “if
\(n\) is even then
\(n^3\) is even” to be true.
3.
For each of the following conditional statements, write a contrapositive.
If the Blue Jays win the World Series, I will eat my hat.
Sharks die if they stop moving.
When the weather is cold, the roads are slippery.
If \(n\) is odd, then it can be written as the sum of an even number and an odd number.
4.
For each of the following conditional statements, determine its truth value. If it is false, make it true by limiting its domain.
If \(p\) is prime, then \(p^2\) is odd.
If \(Q\) is a polygon, then the sum of its interior angles is at least \(360^\circ\text{.}\)
If person \(A\) speaks both official languages of Canada, then they are bilingual.
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5.
Prove that if
\(a, b, c, d\) are all odd, then their sum or their product is odd.
6.
Which of the following are biconditional statements?
If the sky is clear at night, you can see the stars.
You will excel academically if and only if you study diligently.
\(2n\) is even exactly when \(n\) is an integer.
If \(x\) is an integer, then there is exactly one solution to \(x^4=1\text{.}\)
If an animal is a bird then it has feathers, and if an animal has feathers then it is a bird.
7.
For each of the following statements, determine whether or not the given value of the variable is a counterexample. If not, find a value that is, and prove that it is a counterexample.
Subsection 1.4.3 Set Theory Questions
Exercises Exercises
1.
For each of the following sets, state a possible universe.
{a, b, c, d, e, f}
\(\displaystyle {1, 2, 3, 5, 1, 4}\)
{Cats, dogs, parrots, goldfish, snakes}
\(\displaystyle {x ∈ ℤ ∣ x \text{ is prime}}\)
{I want to go to the park.}
2.
Rewrite each of the following sets by listing their members.
{All egg-laying mammals}
{All Canadian provinces}
\(\displaystyle ∅\)
{All prime numbers between 10 and 30}
\(\displaystyle {x ∈ ℚ ∣ |x| < 4}\)
{Three NHL teams}
3.
Using the following sets, write the resulting set of each of the set operations below:
\(A = {1, 2, 3, 7, 8}\) \(B = {4, 9, 1, 0}\) \(C = {1^4, 2^2, (7-2), 10}\) \(D = {6}\)
\(\displaystyle A ∪ B\)
\(\displaystyle B ∩ C\)
\(\displaystyle A ∩ B ∩ C\)
\(\displaystyle B - A\)
4.
Create a set with cardinality 20 in the universe
\(ℕ₀\text{.}\)
5.
Within the universe
\(ℤ\text{,}\) find
\(∼ℕ\text{.}\)
6.
Prove that for any two sets
\(A, B\text{,}\) \(A ∩ B\) is a subset of both
\(A\) and
\(B\text{.}\) Is the same true of
\(A ∪ B\text{?}\) If so, prove it. If not, provide a counterexample.
7.
If
\(A - B = {x = 2n ∣ n ∈ ℕ₀}\text{,}\) then for each of the following values of
\(A\text{,}\) find
\(B\text{.}\)
\(\displaystyle A = ℤ\)
\(A = \mathbb{N}_0\) …
\(A = \mathbb{N}\) …
\(A = \mathbb{Q}\) …
8.
What is the cardinality of each of the following sets?
\(\displaystyle \{corn, peas, carrots, potatoes, tomatoes, pumpkins\}\)
\(\displaystyle \{x ∣ x \text{ is a perfect square and } x ≤ 100\}\)
\(\displaystyle \{x \ | \ x \text{is a two digit natural number with no digit being 7 and } x \leq 100 \}\)
\(\displaystyle \{40, 45, 50, 55, \ldots, 375, 380, 385 \} \)
Subsection 1.4.4 Explanatory and Critical Thinking Questions
Exercises Exercises
1.
A student says: “The sentence ‘This statement is false’ must be false, because it says so.” Do you agree? Explain what makes this sentence different from other statements in logic.
2.
Another student argues: “Questions and commands are not statements, because they don’t have truth values. But they are still useful in logic because we can always assign them ‘true’ or ‘false.’” Identify the flaw in this reasoning.
3.
A student claims: “The contrapositive of ‘If a shape is a square, then it has four sides’ is ‘If a shape is not a square, then it does not have four sides.’” Is this correct? If not, explain why and give the correct contrapositive.
4.
Consider the statement: “If n is even, then n² is divisible by 4.” A student says: “This is true because 2² = 4 and 4 is divisible by 4.” Critique the student’s reasoning. What mistake are they making, and how should the statement be proved?
5.
A student writes: “The set {1, 2, 2, 3} has four elements.” Explain why this is incorrect, and clarify how sets treat repeated elements.
6.
Another student argues: “The empty set must contain something — otherwise, how would we know it exists?” Do you agree or disagree? Explain carefully.
7.
Suppose a student claims: “Since A ∩ B is always a subset of A and B, then A ∪ B must always be a subset of both as well.” Explain why this reasoning is flawed, and give a counterexample.