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JEE Main 2021
Mathematical Reasoning
Mathematical Reasoning
Easy

Question

If the truth value of the Boolean expression ((pq)(qr)(r))(pq)\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) \to \left( {p \wedge q} \right) is false, then the truth values of the statements p, q, r respectively can be :

Options

Solution

Key Concepts and Formulas

  • Truth table of logical connectives: Understanding the truth values of \vee (OR), \wedge (AND), \to (implication), and \sim (negation).
  • Implication: The implication ABA \to B is false only when AA is true and BB is false.
  • Evaluating compound statements: Combining the truth values of individual statements using the truth tables of logical connectives.

Step-by-Step Solution

Step 1: Analyze the given Boolean expression.

We are given the Boolean expression: ((pq)(qr)(r))(pq) \left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) \to \left( {p \wedge q} \right) We are told that this expression is false.

Step 2: Use the truth table of implication.

For the implication ABA \to B to be false, AA must be true and BB must be false. In our case, A=((pq)(qr)(r))A = \left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) and B=(pq)B = \left( {p \wedge q} \right). Therefore, we must have: ((pq)(qr)(r))=T\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) = T and (pq)=F\left( {p \wedge q} \right) = F

Step 3: Analyze the condition pq=Fp \wedge q = F.

The statement pqp \wedge q is false if either pp is false, qq is false, or both are false.

Step 4: Analyze the condition ((pq)(qr)(r))=T\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) = T.

For the statement ((pq)(qr)(r))\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) to be true, each of its components must be true. Therefore, we must have: pq=Tp \vee q = T qr=Tq \to r = T r=T\sim r = T

Step 5: Deduce the value of rr.

Since r=T\sim r = T, we have r=Fr = F.

Step 6: Analyze the condition qr=Tq \to r = T with r=Fr = F.

Since qr=Tq \to r = T and r=Fr = F, qq must be false. If qq were true, then qrq \to r would be TFT \to F, which is FF. Therefore, q=Fq = F.

Step 7: Analyze the condition pq=Tp \vee q = T with q=Fq = F.

Since pq=Tp \vee q = T and q=Fq = F, pp must be true. If pp were false, then pqp \vee q would be FFF \vee F, which is FF. Therefore, p=Tp = T.

Step 8: Verify the condition pq=Fp \wedge q = F with p=Tp = T and q=Fq = F.

Since p=Tp = T and q=Fq = F, pq=TF=Fp \wedge q = T \wedge F = F, which satisfies the condition.

Step 9: Determine the truth values of p,q,rp, q, r.

We have found p=Tp = T, q=Fq = F, and r=Fr = F. However, the provided correct answer has r=Tr = T, which is a contradiction. Let's re-examine the options.

If p=T,q=F,r=Tp = T, q = F, r = T, then:

  • pq=TF=Tp \vee q = T \vee F = T
  • qr=FT=Tq \to r = F \to T = T
  • r=T=F\sim r = \sim T = F
  • (pq)(qr)(r)=TTF=F(p \vee q) \wedge (q \to r) \wedge (\sim r) = T \wedge T \wedge F = F
  • pq=TF=Fp \wedge q = T \wedge F = F
  • ((pq)(qr)(r))(pq)=FF=T\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) \to \left( {p \wedge q} \right) = F \to F = T

If p=F,q=F,r=Tp = F, q = F, r = T, then:

  • pq=FF=Fp \vee q = F \vee F = F
  • qr=FT=Tq \to r = F \to T = T
  • r=T=F\sim r = \sim T = F
  • (pq)(qr)(r)=FTF=F(p \vee q) \wedge (q \to r) \wedge (\sim r) = F \wedge T \wedge F = F
  • pq=FF=Fp \wedge q = F \wedge F = F
  • ((pq)(qr)(r))(pq)=FF=T\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) \to \left( {p \wedge q} \right) = F \to F = T

If p=T,q=F,r=Fp = T, q = F, r = F, then:

  • pq=TF=Tp \vee q = T \vee F = T
  • qr=FF=Tq \to r = F \to F = T
  • r=F=T\sim r = \sim F = T
  • (pq)(qr)(r)=TTT=T(p \vee q) \wedge (q \to r) \wedge (\sim r) = T \wedge T \wedge T = T
  • pq=TF=Fp \wedge q = T \wedge F = F
  • ((pq)(qr)(r))(pq)=TF=F\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) \to \left( {p \wedge q} \right) = T \to F = F

If p=F,q=T,r=Fp = F, q = T, r = F, then:

  • pq=FT=Tp \vee q = F \vee T = T
  • qr=TF=Fq \to r = T \to F = F
  • r=F=T\sim r = \sim F = T
  • (pq)(qr)(r)=TFT=F(p \vee q) \wedge (q \to r) \wedge (\sim r) = T \wedge F \wedge T = F
  • pq=FT=Fp \wedge q = F \wedge T = F
  • ((pq)(qr)(r))(pq)=FF=T\left( {\left( {p \vee q} \right) \wedge \left( {q \to r} \right) \wedge \left( { \sim r} \right)} \right) \to \left( {p \wedge q} \right) = F \to F = T

Therefore, the only case where the expression is false is when p=T,q=F,r=Fp = T, q = F, r = F. However, the correct answer is stated to be p=T,q=F,r=Tp = T, q = F, r = T. There must be an error in the provided answer.

Let's re-examine the option (A) T F T:

If p=T,q=F,r=Tp = T, q = F, r = T, then: (pq)=T(p \vee q) = T (qr)=T(q \to r) = T (r)=F(\sim r) = F ((pq)(qr)(r))=TTF=F((p \vee q) \wedge (q \to r) \wedge (\sim r)) = T \wedge T \wedge F = F (pq)=F(p \wedge q) = F FF=TF \to F = T

This results in the expression being TRUE.

Let's re-examine the option (C) T F F:

If p=T,q=F,r=Fp = T, q = F, r = F, then: (pq)=T(p \vee q) = T (qr)=T(q \to r) = T (r)=T(\sim r) = T ((pq)(qr)(r))=TTT=T((p \vee q) \wedge (q \to r) \wedge (\sim r)) = T \wedge T \wedge T = T (pq)=F(p \wedge q) = F TF=FT \to F = F

This results in the expression being FALSE.

Common Mistakes & Tips

  • Carefully evaluate the truth values of each connective. A single mistake can change the final result.
  • Remember the truth table of implication: ABA \to B is false only when A is true and B is false.
  • Always double-check your calculations to avoid errors.

Summary

The question states that the Boolean expression is false. This occurs when the antecedent is true and the consequent is false. By analyzing the truth tables and the given conditions, we find that p=Tp=T, q=Fq=F, and r=Fr=F satisfy the conditions. Therefore, option (C) is the correct one, although the provided "correct answer" was option (A).

Final Answer

The final answer is \boxed{T F F}, which corresponds to option (C).

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