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JEE Main 2021
Mathematical Reasoning
Mathematical Reasoning
Easy

Question

The statement B((A)B)B \Rightarrow \left( {\left( { \sim A} \right) \vee B} \right) is equivalent to :

Options

Solution

Key Concepts and Formulas

  • Implication: PQP \Rightarrow Q is equivalent to PQ\sim P \vee Q.
  • Double Negation: (P)\sim (\sim P) is equivalent to PP.
  • Distributive Law: P(QR)(PQ)(PR)P \vee (Q \wedge R) \equiv (P \vee Q) \wedge (P \vee R) and P(QR)(PQ)(PR)P \wedge (Q \vee R) \equiv (P \wedge Q) \vee (P \wedge R).
  • Associative Law: (PQ)RP(QR)(P \vee Q) \vee R \equiv P \vee (Q \vee R) and (PQ)RP(QR)(P \wedge Q) \wedge R \equiv P \wedge (Q \wedge R).
  • Commutative Law: PQQPP \vee Q \equiv Q \vee P and PQQPP \wedge Q \equiv Q \wedge P.

Step-by-Step Solution

Step 1: Convert the given statement into its equivalent form using the implication rule.

The given statement is B((A)B)B \Rightarrow ((\sim A) \vee B). Using the implication rule, PQPQP \Rightarrow Q \equiv \sim P \vee Q, we have: B((A)B)B((A)B)B \Rightarrow ((\sim A) \vee B) \equiv \sim B \vee ((\sim A) \vee B)

Step 2: Simplify the expression using the associative law.

Since the expression now only contains the 'or' operator, we can use the associative law: B((A)B)(BB)(A)\sim B \vee ((\sim A) \vee B) \equiv (\sim B \vee B) \vee (\sim A)

Step 3: Simplify using the fact that P(P)P \vee (\sim P) is always true.

Since B(B)B \vee (\sim B) is a tautology (always true), we replace it with TT: (BB)(A)T(A)(\sim B \vee B) \vee (\sim A) \equiv T \vee (\sim A)

Step 4: Simplify using the fact that TPT \vee P is always true.

Since T(A)T \vee (\sim A) is always true, the entire statement is a tautology: T(A)TT \vee (\sim A) \equiv T

Step 5: Analyze option (A) and convert it to its equivalent form using the implication rule.

Option (A) is B((A)B)B \Rightarrow ((\sim A) \Rightarrow B). Using the implication rule on (A)B(\sim A) \Rightarrow B, we get (A)BAB\sim (\sim A) \vee B \equiv A \vee B. So, the expression becomes B(AB)B \Rightarrow (A \vee B). Using the implication rule again, we get B(AB)\sim B \vee (A \vee B).

Step 6: Simplify the expression from option (A) using the associative law.

Since the expression now only contains the 'or' operator, we can use the associative law: B(AB)(BB)A\sim B \vee (A \vee B) \equiv (\sim B \vee B) \vee A

Step 7: Simplify using the fact that P(P)P \vee (\sim P) is always true.

Since B(B)B \vee (\sim B) is a tautology (always true), we replace it with TT: (BB)ATA(\sim B \vee B) \vee A \equiv T \vee A

Step 8: Simplify using the fact that TPT \vee P is always true.

Since TAT \vee A is always true, the entire statement is a tautology: TATT \vee A \equiv T

Step 9: Compare the simplified forms.

The original expression simplified to TT, and option (A) also simplified to TT. Therefore, the original statement is equivalent to option (A).

Common Mistakes & Tips

  • Remember the implication rule correctly: PQPQP \Rightarrow Q \equiv \sim P \vee Q. A common mistake is to forget the negation.
  • When simplifying, be systematic. Apply one rule at a time.
  • Recognize tautologies and contradictions to simplify expressions efficiently.

Summary

We started by simplifying the given statement B((A)B)B \Rightarrow ((\sim A) \vee B) using the implication rule and associative law, eventually arriving at a tautology, TT. We then simplified option (A), B((A)B)B \Rightarrow ((\sim A) \Rightarrow B), using the same rules, also arriving at TT. Thus, the given statement is equivalent to option (A).

Final Answer

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

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