Which of the following is not a tautology?

This question was previously asked in
HTET PGT Official Computer Science Paper - 2017
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  1. P ⋁ ¬ P 
  2. ( P ⋀ Q) ⇒ P
  3. (P ⇒ Q) ⋁ P 

  4. (P ⋁ Q) ⇒ P

Answer (Detailed Solution Below)

Option 4 : (P ⋁ Q) ⇒ P
Free
HTET PGT Official Computer Science Paper - 2019
4.4 K Users
60 Questions 60 Marks 60 Mins

Detailed Solution

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Option (i)

P

Q

¬ P

P v ¬p

T

T

F

T

T

F

F

T

F

T

T

T

F

F

T

T

Option (ii)

P

Q

P ⋀ Q

P ⋀ Q  ⇒ P

T

T

T

T

T

F

F

T

F

T

F

T

F

F

F

T

Option (iii)

P

Q

P ⇒ Q

(P  ⇒ Q) ⋁ P

T

T

T

T

T

F

F

T

F

T

T

T

F

F

T

T

Option (iv)

P

Q

P V Q

(P ⋁ Q) ⇒ P

T

T

T

T

T

F

T

T

F

T

T

F

F

F

F

T

 

Therefore it  is not a tautology Option 4 is the answer

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