Logical Connectives - Expert Level: disjunction EXPERT

Step 1: Understand necessary and sufficient conditions
• P is NECESSARY for Q: Q cannot be true without P (Q → P)
• P is SUFFICIENT for Q: P being true guarantees Q (P → Q)
• P is BOTH: P if and only if Q (P ↔ Q)

Step 2: Analyze the relationship
P: Studying
Q: Passing the exam

Step 3: Determine the condition type
You need to study to pass (necessary), but studying alone doesn't guarantee passing (not sufficient)

Answer: Necessary but not sufficient

Step 1: Understand 'neither...nor' statements
'Neither p nor q' means 'not p AND not q'
Logical form: ¬p ∧ ¬q

Step 2: Build the truth table
¬p ∧ ¬q is True ONLY when both p and q are False
Truth table:
p=T, q=T → ¬T ∧ ¬T = F ∧ F = F
p=T, q=F → ¬T ∧ ¬F = F ∧ T = F
p=F, q=T → ¬F ∧ ¬T = T ∧ F = F
p=F, q=F → ¬F ∧ ¬F = T ∧ T = T

Step 3: Apply given values
p = F, q = F
¬p = T, ¬q = T
¬p ∧ ¬q = T

Answer: True

Step 1: Understand Exclusive OR (XOR)
Exclusive OR (p ⊕ q) is True when EXACTLY ONE of p or q is True.
It is False when both are True or both are False.

Truth table for p ⊕ q:
p=T, q=T → Result=F (both true)
p=T, q=F → Result=T (exactly one)
p=F, q=T → Result=T (exactly one)
p=F, q=F → Result=F (neither true)

Step 2: Apply the given values
p = True, q = True

Step 3: Evaluate the exclusive OR
Since both have the same truth value, the exclusive OR is False

Step 1: Understand 'only if' statements
'P only if Q' means 'If P, then Q' (P → Q)
This is DIFFERENT from 'If Q then P' (Q → P)

Key insight: 'only if' introduces a NECESSARY condition
Q is necessary for P (P cannot be true without Q)

Step 2: Identify components
Statement: A number is prime only if it is greater than 1
P: Number is prime, G: Greater than 1

Step 3: Convert to logical form
Logical form: P → G
Equivalent: If a number is prime, then it is greater than 1

Step 4: Important distinction
Note: Being greater than 1 is necessary but not sufficient
'Only if' ≠ 'If and only if'
'Only if' gives one direction only (→)
'If and only if' gives both directions (↔)

Rule: Disjunctive Syllogism

If P ∨ Q is true and P is false, then Q must be true.

Answer: q

Implication is not symmetric. Converse is not equivalent to original.

p=True, q=False: p→q=False, q→p=True → Different!

Step 1: Understand the conjunction (AND) operator
The conjunction p ∧ q is True ONLY when BOTH p and q are True.

Step 2: Apply the given values
p = False, q = False

Step 3: Evaluate p ∧ q
Since at least one of p or q is False, p ∧ q = False

Step 1: Understand the conjunction (AND) operator
The conjunction p ∧ q is True ONLY when BOTH p and q are True.

Step 2: Apply the given values
p = True, q = True

Step 3: Evaluate p ∧ q
Since both p and q are True, p ∧ q = True

Rule: Modus Tollens

If P → Q and Q is false, then P must be false.

Answer: ¬p

Step 1: Analyze A's statement
A says: 'We are both knaves'

Step 2: Test if A is a knight
If A is a knight, then A tells the truth.
But then 'We are both knaves' would be true.
This means A is a knave, which contradicts our assumption.
Therefore, A cannot be a knight.

Step 3: Test if A is a knave
If A is a knave, then A lies.
A's statement 'We are both knaves' must be false.
For 'both knaves' to be false, at least one must be a knight.
Since A is a knave, B must be a knight.

Step 4: Verify
A (knave) lies: 'We are both knaves' is indeed false ✓
B is a knight ✓

Answer: A is a knave, B is a knight

Step 1: Understand 'only if' statements
'P only if Q' means 'If P, then Q' (P → Q)
This is DIFFERENT from 'If Q then P' (Q → P)

Key insight: 'only if' introduces a NECESSARY condition
Q is necessary for P (P cannot be true without Q)

Step 2: Identify components
Statement: The alarm rings only if there is an intruder
A: Alarm rings, I: There is an intruder

Step 3: Convert to logical form
Logical form: A → I
Equivalent: If the alarm rings, then there is an intruder

Step 4: Important distinction
Note: Intruder is necessary but not sufficient
'Only if' ≠ 'If and only if'
'Only if' gives one direction only (→)
'If and only if' gives both directions (↔)

Equal sets mean element is in A IF AND ONLY IF it is in B, which is biconditional.

Answer: x ∈ A ↔ x ∈ B

Step 1: Understand 'unless' statements
'P unless Q' means 'If not Q, then P'
In logical form: 'P unless Q' ≡ '¬Q → P'

Step 2: Identify components
Original: You will fail unless you study
p: You will fail, q: You study

Step 3: Convert to if-then form
'unless' tells us what happens if the condition is NOT met
Logical form: ¬q → p

Step 4: Write in English
Equivalent statement: If you do not study, then you will fail

'If P then Q' translates to P → Q (implication).

Answer: p → q

Step 1: Understand 'unless' statements
'P unless Q' means 'If not Q, then P'
In logical form: 'P unless Q' ≡ '¬Q → P'

Step 2: Identify components
Original: She won't come unless you invite her
p: She won't come, q: You invite her

Step 3: Convert to if-then form
'unless' tells us what happens if the condition is NOT met
Logical form: ¬q → p

Step 4: Write in English
Equivalent statement: If you do not invite her, then she won't come

Step 1: Understand the classifications
• Tautology: Always True for all possible truth values
• Contradiction: Always False for all possible truth values
• Contingent: True for some values, False for others

Step 2: Analyze the expression
Expression: p → p

Step 3: Test all possible combinations
Truth table:
p=T: T → T = T
p=F: F → F = T
Result: Always True → TAUTOLOGY
This is the Law of Identity

Step 1: Understand the implication (→) operator
The implication p → q is False ONLY when p is True but q is False.
In all other cases, it is True.

Truth table for p → q:
p=T, q=T → Result=T
p=T, q=F → Result=F (the only False case)
p=F, q=T → Result=T
p=F, q=F → Result=T

Step 2: Apply the given values
p = True, q = True

Step 3: Evaluate p → q
Since this is not the case where p is True and q is False, p → q = True

Valid: Some A are B, no B are C → Some A are not C.

Step 1: Understand what logical equivalence means
Two expressions are logically equivalent if they have the same truth value for ALL possible combinations of variables.

Step 2: Analyze the expressions
Expression 1: ¬(p ∨ q)
Expression 2: ¬p ∧ ¬q

Step 3: Apply De Morgan's Law
De Morgan's Law states: ¬(p ∨ q) ≡ ¬p ∧ ¬q
The negation of a disjunction equals the conjunction of negations.
These expressions ARE equivalent.

Step 1: Understand the biconditional (↔) operator
The biconditional p ↔ q is True when BOTH p and q have the SAME truth value.
It is False when p and q have DIFFERENT truth values.

Truth table for p ↔ q:
p=T, q=T → Result=T (same)
p=T, q=F → Result=F (different)
p=F, q=T → Result=F (different)
p=F, q=F → Result=T (same)

Step 2: Apply the given values
p = True, q = False

Step 3: Evaluate p ↔ q
Since p and q have different truth values, p ↔ q = False