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JEE Main 2019
Mathematical Reasoning
Mathematical Reasoning
Medium

Question

Let p and q be two statements. Then (p(pq)) \sim \left( {p \wedge (p \Rightarrow \, \sim q)} \right) is equivalent to

Options

Solution

Key Concepts and Formulas

  • Implication: pqpqp \Rightarrow q \equiv \sim p \vee q
  • De Morgan's Laws:
    • (pq)pq\sim (p \wedge q) \equiv \sim p \vee \sim q
    • (pq)pq\sim (p \vee q) \equiv \sim p \wedge \sim q
  • Distributive Law: p(qr)(pq)(pr)p \vee (q \wedge r) \equiv (p \vee q) \wedge (p \vee r)
  • Tautology: ppTp \vee \sim p \equiv T (where T represents True)
  • Identity Law: pTpp \wedge T \equiv p

Step-by-Step Solution

Step 1: Rewrite the implication using the equivalence pqpqp \Rightarrow q \equiv \sim p \vee q. We have (p(pq))=(p(pq)) \sim (p \wedge (p \Rightarrow \sim q)) = \sim (p \wedge (\sim p \vee \sim q)) We are replacing pqp \Rightarrow \sim q with pq\sim p \vee \sim q.

Step 2: Apply De Morgan's Law to the outer negation. (p(pq))=p(pq) \sim (p \wedge (\sim p \vee \sim q)) = \sim p \vee \sim (\sim p \vee \sim q) We are using (pq)=pq\sim (p \wedge q) = \sim p \vee \sim q, where qq is (pq)(\sim p \vee \sim q).

Step 3: Apply De Morgan's Law again to the inner negation. p(pq)=p(pq) \sim p \vee \sim (\sim p \vee \sim q) = \sim p \vee (p \wedge q) We are using (pq)=pq\sim (p \vee q) = \sim p \wedge \sim q, where pp is p\sim p and qq is q\sim q. Thus (pq)=(p)(q)=pq\sim(\sim p \vee \sim q) = \sim(\sim p) \wedge \sim(\sim q) = p \wedge q.

Step 4: Apply the distributive law. p(pq)=(pp)(pq) \sim p \vee (p \wedge q) = (\sim p \vee p) \wedge (\sim p \vee q) We are using a(bc)=(ab)(ac)a \vee (b \wedge c) = (a \vee b) \wedge (a \vee c), where aa is p\sim p, bb is pp, and cc is qq.

Step 5: Simplify using the tautology ppTp \vee \sim p \equiv T. (pp)(pq)=T(pq) (\sim p \vee p) \wedge (\sim p \vee q) = T \wedge (\sim p \vee q) Since pp\sim p \vee p is always true, we replace it with TT.

Step 6: Simplify using the identity law pTpp \wedge T \equiv p. T(pq)=pq T \wedge (\sim p \vee q) = \sim p \vee q Since T(pq)T \wedge (\sim p \vee q) is logically equivalent to pq\sim p \vee q.

Common Mistakes & Tips

  • Be careful when applying De Morgan's Laws. Remember to negate all terms inside the parentheses and change the operator.
  • When simplifying logical expressions, it's often helpful to rewrite implications using their equivalent forms with negations and disjunctions.
  • Use the distributive law carefully. Ensure you understand how it works with both conjunctions and disjunctions.

Summary

We started with the expression (p(pq))\sim (p \wedge (p \Rightarrow \sim q)) and simplified it using logical equivalences. We first replaced the implication with its equivalent form using disjunctions and negations. Then, we applied De Morgan's Laws and the distributive law to arrive at the simplified expression pq\sim p \vee q. This expression is logically equivalent to the original expression.

Final Answer The final answer is pq\boxed{\sim p \vee q}, which corresponds to option (A).

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