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JEE Main 2021
Mathematical Reasoning
Mathematical Reasoning
Easy

Question

Consider : Statement − I : (pq)(pq)\left( {p \wedge \sim q} \right) \wedge \left( { \sim p \wedge q} \right) is a fallacy. Statement − II : (pq)(qp)\left( {p \to q} \right) \leftrightarrow \left( { \sim q \to \sim p} \right) is a tautology.

Options

Solution

Key Concepts and Formulas

  • Logical Connectives: Understanding the truth tables for AND (\wedge), OR (\vee), NOT (\sim), implication (\to), and bi-conditional (\leftrightarrow).
  • Tautology: A statement that is always true, regardless of the truth values of its components.
  • Fallacy (Contradiction): A statement that is always false, regardless of the truth values of its components.
  • Implication: pqp \to q is equivalent to pq\sim p \vee q.
  • Bi-conditional: pqp \leftrightarrow q is equivalent to (pq)(qp)(p \to q) \wedge (q \to p).

Step-by-Step Solution

Statement I Analysis:

Step 1: Create a truth table for the expression (pq)(pq)(p \wedge \sim q) \wedge (\sim p \wedge q).

We need to consider all possible combinations of truth values for pp and qq.

Step 2: Determine the truth values of p\sim p and q\sim q.

If pp is true, then p\sim p is false, and vice-versa. Similarly, if qq is true, then q\sim q is false, and vice-versa.

Step 3: Determine the truth values of pqp \wedge \sim q.

pqp \wedge \sim q is true only when both pp and q\sim q are true.

Step 4: Determine the truth values of pq\sim p \wedge q.

pq\sim p \wedge q is true only when both p\sim p and qq are true.

Step 5: Determine the truth values of (pq)(pq)(p \wedge \sim q) \wedge (\sim p \wedge q).

(pq)(pq)(p \wedge \sim q) \wedge (\sim p \wedge q) is true only when both pqp \wedge \sim q and pq\sim p \wedge q are true.

The truth table is as follows:

ppqqp\sim pq\sim qpqp \wedge \sim qpq\sim p \wedge q(pq)(pq)(p \wedge \sim q) \wedge (\sim p \wedge q)
TTFFFFF
TFFTTFF
FTTFFTF
FFTTFFF

Step 6: Analyze the final column.

Since the last column is always false, the expression (pq)(pq)(p \wedge \sim q) \wedge (\sim p \wedge q) is a fallacy. Therefore, Statement I is true.

Statement II Analysis:

Step 1: Create a truth table for the expression (pq)(qp)(p \to q) \leftrightarrow (\sim q \to \sim p).

Step 2: Determine the truth values of p\sim p and q\sim q.

Step 3: Determine the truth values of pqp \to q.

pqp \to q is false only when pp is true and qq is false. Otherwise, it is true. Equivalently, pqpqp \to q \equiv \sim p \vee q.

Step 4: Determine the truth values of qp\sim q \to \sim p.

qp\sim q \to \sim p is false only when q\sim q is true and p\sim p is false (i.e., qq is false and pp is true). Otherwise, it is true. Equivalently, qpqp\sim q \to \sim p \equiv q \vee \sim p.

Step 5: Determine the truth values of (pq)(qp)(p \to q) \leftrightarrow (\sim q \to \sim p).

(pq)(qp)(p \to q) \leftrightarrow (\sim q \to \sim p) is true when both pqp \to q and qp\sim q \to \sim p have the same truth value. It is false when they have different truth values.

The truth table is as follows:

ppqqp\sim pq\sim qpqp \to qqp\sim q \to \sim p(pq)(qp)(p \to q) \leftrightarrow (\sim q \to \sim p)
TTFFTTT
TFFTFFT
FTTFTTT
FFTTTTT

Step 6: Analyze the final column.

Since the last column is always true, the expression (pq)(qp)(p \to q) \leftrightarrow (\sim q \to \sim p) is a tautology. Therefore, Statement II is true.

Step 7: Determine if Statement II is a correct explanation for Statement I.

Statement II is about the equivalence of an implication and its contrapositive. Statement I is about a compound statement that is always false. There is no direct relationship between the two. Therefore, Statement II is not a correct explanation for Statement I.

Common Mistakes & Tips

  • Be careful with the order of operations when evaluating logical expressions. Parentheses are important.
  • Remember the truth tables for all the logical connectives.
  • When proving a statement is a tautology or fallacy, it is usually sufficient to create a truth table.
  • pqp \to q is only false when pp is true and qq is false.

Summary

Statement I is a fallacy, and Statement II is a tautology. Statement II does not explain Statement I. Therefore, Statement I is True; Statement II is True; Statement II is not a correct explanation for Statement I.

Final Answer

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

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