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JEE Main 202118 MarchEvening Shift
Exam
JEE Main
Year
2021
Date
18 March
Shift
Evening Shift
Subject
Mathematics
Chapter
Mathematical Reasoning

JEE Main 2021 · 18 March · Evening Shift · Mathematics · Mathematical Reasoning

If P and Q are two statements, then which of the following compound...

If PP and QQ are two statements, then which of the following compound statement is a tautology?If P and Q are two statements, then which of the following compound statement is a tautology?

Options

  1. A((P⇒Q)∧ Q)⇒Q( ( P ⇒ Q ) ∧ ~ Q ) ⇒ Q
  2. B((P⇒Q)∧ Q)⇒ P( ( P ⇒ Q ) ∧ ~ Q ) ⇒ ~ P
  3. C((P⇒Q)∧ Q)⇒P( ( P ⇒ Q ) ∧ ~ Q ) ⇒ P
  4. D((P⇒Q)∧ Q)⇒(P∧Q)( ( P ⇒ Q ) ∧ ~ Q ) ⇒ ( P ∧ Q )

Correct answer

B. ((P⇒Q)∧ Q)⇒ P( ( P ⇒ Q ) ∧ ~ Q ) ⇒ ~ P

Step-by-step solution

LHS of all the options are same i.e. ((P→Q)∧ Q)( ( P \to Q ) ∧ ~ Q )

≡( P∨Q)∧ Q≡ ( ~ P ∨ Q ) ∧ ~ Q

≡( P∧ Q)∨(Q∧ Q)≡ ( ~ P ∧ ~ Q ) ∨ ( Q ∧ ~ Q )

≡ P∧ Q≡ ~ P ∧ ~ Q

(A) ( P∧ Q)→Q( ~ P ∧ ~ Q ) \to Q

≡ ( P∧ Q)∨Q≡ ~ ( ~ P ∧ ~ Q ) ∨ Q

≡(P∨Q)∨Q≠≡ ( P ∨ Q ) ∨ Q \ne tautology

(B) ( P∧ Q)→ P( ~ P ∧ ~ Q ) \to ~ P

≡ ( P∧ Q)∨ P≡ ~ ( ~ P ∧ ~ Q ) ∨ ~ P

≡(P∨Q)∨ P≡ ( P ∨ Q ) ∨ ~ P

If P and Q are two statements, then which of the following compound statement is a tautology?

(C) ( P∧ Q)→P( ~ P ∧ ~ Q ) \to P

≡(P∨Q)∨P≠≡ ( P ∨ Q ) ∨ P \ne Tautology

(D) ( P∧ Q)→(P∧Q)( ~ P ∧ ~ Q ) \to ( P ∧ Q )

≡(P∨Q)∨(P∧Q)≠≡ ( P ∨ Q ) ∨ ( P ∧ Q ) \ne Tautology

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