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JEE Main 2021
Mathematical Reasoning
Mathematical Reasoning
Easy

Question

If (p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q is false, then the truth values of p,qp, q and rr are, respectively :

Options

Solution

Key Concepts and Formulas

  • Implication (→): ABA \to B is false only when AA is true and BB is false. Otherwise, it is true.
  • Conjunction (∧): ABA \wedge B is true only when both AA and BB are true. Otherwise, it is false.
  • Disjunction (∨): ABA \vee B is false only when both AA and BB are false. Otherwise, it is true.
  • Negation (∼): If AA is true, then A\sim A is false, and if AA is false, then A\sim A is true.

Step-by-Step Solution

Step 1: Analyze the given statement. We are given that (pq)(pr)pq(p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q is false. Using the truth table properties of implication, the only way an implication ABA \to B can be false is when AA is true and BB is false. Therefore, we must have: (pq)(pr) is True, and pq is False.(p \wedge \sim q) \wedge (p \wedge r) \text{ is True, and } \sim p \vee q \text{ is False.}

Step 2: Determine the truth values of p\sim p and qq. Since pq\sim p \vee q is false, both p\sim p and qq must be false. p=False and q=False.\sim p = \text{False} \text{ and } q = \text{False}.

Step 3: Determine the truth value of pp. Since p\sim p is false, pp must be true. p=True.p = \text{True}.

Step 4: Determine the truth value of q\sim q. Since qq is false, q\sim q must be true. q=True.\sim q = \text{True}.

Step 5: Analyze the truth values of pqp \wedge \sim q and prp \wedge r. Since (pq)(pr)(p \wedge \sim q) \wedge (p \wedge r) is true, both pqp \wedge \sim q and prp \wedge r must be true. pq=True and pr=True.p \wedge \sim q = \text{True} \text{ and } p \wedge r = \text{True}.

Step 6: Determine the truth value of rr. We know p=Truep = \text{True}, and pr=Truep \wedge r = \text{True}. Therefore, rr must be true. r=True.r = \text{True}.

Step 7: Summarize the truth values. We have found that p=Truep = \text{True}, q=Falseq = \text{False}, and r=Truer = \text{True}.

Step 8: Check if the solution matches the given correct answer. The truth values we found are p=T,q=F,r=Tp = T, q = F, r = T. The given correct answer is (A) F, T, F. However, we derived T, F, T. There seems to be a mistake in the provided correct answer. Let's re-evaluate the given options. If p=F,q=T,r=Fp=F, q=T, r=F (option A), then q=F\sim q = F, so pq=FF=Fp \wedge \sim q = F \wedge F = F. Therefore (pq)(pr)=F(FF)=F(p \wedge \sim q) \wedge (p \wedge r) = F \wedge (F \wedge F) = F. Also, pq=TT=T\sim p \vee q = T \vee T = T. So, (pq)(pr)pq(p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q becomes FTF \to T which is True.

If p=T,q=F,r=Tp=T, q=F, r=T (option B), then q=T\sim q = T, so pq=TT=Tp \wedge \sim q = T \wedge T = T. Therefore (pq)(pr)=T(TT)=T(p \wedge \sim q) \wedge (p \wedge r) = T \wedge (T \wedge T) = T. Also, pq=FF=F\sim p \vee q = F \vee F = F. So, (pq)(pr)pq(p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q becomes TFT \to F which is False.

If p=T,q=T,r=Tp=T, q=T, r=T (option C), then q=F\sim q = F, so pq=TF=Fp \wedge \sim q = T \wedge F = F. Therefore (pq)(pr)=F(TT)=F(p \wedge \sim q) \wedge (p \wedge r) = F \wedge (T \wedge T) = F. Also, pq=FT=T\sim p \vee q = F \vee T = T. So, (pq)(pr)pq(p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q becomes FTF \to T which is True.

If p=F,q=F,r=Fp=F, q=F, r=F (option D), then q=T\sim q = T, so pq=FT=Fp \wedge \sim q = F \wedge T = F. Therefore (pq)(pr)=F(FF)=F(p \wedge \sim q) \wedge (p \wedge r) = F \wedge (F \wedge F) = F. Also, pq=TF=T\sim p \vee q = T \vee F = T. So, (pq)(pr)pq(p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q becomes FTF \to T which is True.

The only option that makes the given statement false is option B.

Common Mistakes & Tips

  • Remember the truth tables for each logical operator (∧, ∨, ∼, →).
  • The implication ABA \to B is only false when A is true and B is false.
  • Work systematically and break down the complex statement into smaller parts.

Summary

We are given that (pq)(pr)pq(p \wedge \sim q) \wedge (p \wedge r) \to \sim p \vee q is false. This means that (pq)(pr)(p \wedge \sim q) \wedge (p \wedge r) is true and pq\sim p \vee q is false. From the second part, we deduce that p\sim p is false and qq is false, which means pp is true. From the first part, since pp is true and pqp \wedge \sim q must be true, then q\sim q must be true, which is consistent with qq being false. Since pp is true and prp \wedge r is true, rr must be true. Therefore, p=T,q=F,r=Tp=T, q=F, r=T. This corresponds to option (B). The given correct answer (A) is incorrect.

Final Answer

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

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