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Which of the following pairs of propositions are not logically equivalent ?


1. ((p → r) ∧ (q → r)) and ((p ∨ q) → r)
2. p ↔ q and (¬ p ↔ ¬ q) 
3. (p → q) ∧ (q → p) and p ↔ q
4. ((p ∧ q) → r ) and ((p → r) ∧ (q → r))

1 Answer

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Correct Answer - Option 4 : ((p ∧ q) → r ) and ((p → r) ∧ (q → r))

The correct answer is option 4.

Option A: ((p → r) ∧ (q → r)) and ((p ∨ q) → r)

Both are equal and it gives the same truth table. Hence it is logically equivalent. 

Option B:  p ↔ q and (¬ p ↔ ¬ q)

(p → q)(q → p) and (¬ p → q)(¬ q →p)

(p+q̅)(q+p̅) and (p+q̅)(q+p̅)

Both are equal and it gives the same truth table. Hence it is logically equivalent. 

Option C:  (p → q) ∧ (q → p) and p ↔ q 

Both tables give equal values. Hence it is logically equivalent.

Option D: ((p ∧ q) → r ) and ((p → r) ∧ (q → r))

P Q R ¬P ¬Q P⇒R Q⇒R (P⇒R)(Q⇒R) PQ PQ⇒R
T T T F F T T T T T
T T F F F F F F T F
T F T F T T T T F T
T F F F T F T F F T
F T T T F T T T F T
F T F T F T F F F T
F F T T T T T T F T
F F F T T T T T F T

The above truth table is not equivalent. Hence the above statement is True, Logically not equivalent. 
∴ Hence the correct answer is  ((p ∧ q) → r ) and ((p → r) ∧ (q → r)).

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