Q61 / 150 · MHT CET 2026 (13 April Shift 2) · Mathematics
Which of the following statements is logically equivalent to $\sim(p \leftrightarrow q)$ ?
- A$\sim p \rightarrow q$
- B$\sim p \leftrightarrow \sim q$
- C$\sim(q \rightarrow \sim p)$
- Correct answerD$p \leftrightarrow \sim q$
Solution
The logical equivalence of the biconditional statement is given by:
$p \leftrightarrow q \equiv (p \rightarrow q) \wedge (q \rightarrow p)$
Taking the negation of both sides:
$\sim(p \leftrightarrow q) \equiv \sim((p \rightarrow q) \wedge (q \rightarrow p))$
Using De Morgan's laws:
$\equiv \sim(p \rightarrow q) \vee \sim(q \rightarrow p)$
Since $p \rightarrow q \equiv \sim p \vee q$, we get:
$\equiv \sim(\sim p \vee q) \vee \sim(\sim q \vee p)$
Applying De Morgan's laws again:
$\equiv (p \wedge \sim q) \vee (q \wedge \sim p)$
Now, evaluating the expression $p \leftrightarrow \sim q$:
$p \leftrightarrow \sim q \equiv (p \rightarrow \sim q) \wedge (\sim q \rightarrow p)$
$\equiv (\sim p \vee \sim q) \wedge (q \vee p)$
Using the distributive law:
$\equiv (\sim p \wedge q) \vee (\sim p \wedge p) \vee (\sim q \wedge q) \vee (\sim q \wedge p)$
Since $\sim p \wedge p \equiv F$ and $\sim q \wedge q \equiv F$:
$\equiv (\sim p \wedge q) \vee (p \wedge \sim q)$
Comparing the two results, we conclude that $\sim(p \leftrightarrow q) \equiv p \leftrightarrow \sim q$.
Answer: $p \leftrightarrow \sim q$
$p \leftrightarrow q \equiv (p \rightarrow q) \wedge (q \rightarrow p)$
Taking the negation of both sides:
$\sim(p \leftrightarrow q) \equiv \sim((p \rightarrow q) \wedge (q \rightarrow p))$
Using De Morgan's laws:
$\equiv \sim(p \rightarrow q) \vee \sim(q \rightarrow p)$
Since $p \rightarrow q \equiv \sim p \vee q$, we get:
$\equiv \sim(\sim p \vee q) \vee \sim(\sim q \vee p)$
Applying De Morgan's laws again:
$\equiv (p \wedge \sim q) \vee (q \wedge \sim p)$
Now, evaluating the expression $p \leftrightarrow \sim q$:
$p \leftrightarrow \sim q \equiv (p \rightarrow \sim q) \wedge (\sim q \rightarrow p)$
$\equiv (\sim p \vee \sim q) \wedge (q \vee p)$
Using the distributive law:
$\equiv (\sim p \wedge q) \vee (\sim p \wedge p) \vee (\sim q \wedge q) \vee (\sim q \wedge p)$
Since $\sim p \wedge p \equiv F$ and $\sim q \wedge q \equiv F$:
$\equiv (\sim p \wedge q) \vee (p \wedge \sim q)$
Comparing the two results, we conclude that $\sim(p \leftrightarrow q) \equiv p \leftrightarrow \sim q$.
Answer: $p \leftrightarrow \sim q$
Hence, the correct answer is option: 'D'.