Sets, Relations & Functions
Mathematical Logic - Tautology
Grade 11

Question:

<p>The following statement \((p \to q) \to [(\sim p \to q) \to q]\) is</p>
<p>a fallacy</p>
<p>a tautology</p>
<p>equivalent to \(\sim p \to q\)</p>
<p>equivalent to \(p \to \sim q\)</p>

Step-by-Step Solution

Key Concept: A statement is a tautology if it is true for all possible truth value assignments of its propositional variables. We need to evaluate $(p \to q) \to [(\sim p \to q) \to q]$ for all combinations of p and q.
<p><strong>Step 1: Set up the truth table.</strong> We have two propositional variables p and q, giving 4 cases to evaluate.</p><p><strong>Step 2: Evaluate for p = T, q = T</strong><br/>• $p \to q = T \to T = T$<br/>• $\sim p = F$, so $\sim p \to q = F \to T = T$<br/>• $(\sim p \to q) \to q = T \to T = T$<br/>• $(p \to q) \to [(\sim p \to q) \to q] = T \to T = T$ ✓</p><p><strong>Step 3: Evaluate for p = T, q = F</strong><br/>• $p \to q = T \to F = F$<br/>• $\sim p = F$, so $\sim p \to q = F \to F = T$<br/>• $(\sim p \to q) \to q = T \to F = F$<br/>• $(p \to q) \to [(\sim p \to q) \to q] = F \to F = T$ ✓</p><p><strong>Step 4: Evaluate for p = F, q = T</strong><br/>• $p \to q = F \to T = T$<br/>• $\sim p = T$, so $\sim p \to q = T \to T = T$<br/>• $(\sim p \to q) \to q = T \to T = T$<br/>• $(p \to q) \to [(\sim p \to q) \to q] = T \to T = T$ ✓</p><p><strong>Step 5: Evaluate for p = F, q = F</strong><br/>• $p \to q = F \to F = T$<br/>• $\sim p = T$, so $\sim p \to q = T \to F = F$<br/>• $(\sim p \to q) \to q = F \to F = T$<br/>• $(p \to q) \to [(\sim p \to q) \to q] = T \to T = T$ ✓</p><p><strong>Step 6: Conclusion.</strong> In all four cases, the statement evaluates to T. Therefore, the given statement is a <strong>tautology</strong>.</p><p><strong>∴ Answer: B</strong></p>
Correct Answer: B

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