(3) Prove the following by contraposition. If n? is not divisible by 3, then n is not divisible by 3.

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Chapter2: Second-order Linear Odes
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**Problem 3: Proof by Contraposition**

**Statement:**

Prove the following by contraposition.

If \( n^2 \) is not divisible by 3, then \( n \) is not divisible by 3.

**Explanation:**

To prove this statement using contraposition, we demonstrate that the contrapositive is true. The contrapositive of the given statement is: 

**If \( n \) is divisible by 3, then \( n^2 \) is divisible by 3.**

**Proof:**

1. Assume that \( n \) is divisible by 3. This means there exists an integer \( k \) such that \( n = 3k \).

2. Calculate \( n^2 \):
   \[
   n^2 = (3k)^2 = 9k^2 = 3(3k^2)
   \]

3. Since \( n^2 = 3(3k^2) \), \( n^2 \) is also divisible by 3.

Thus, we have shown that if \( n \) is divisible by 3, then \( n^2 \) is divisible by 3, proving the contrapositive and therefore the original statement.
Transcribed Image Text:**Problem 3: Proof by Contraposition** **Statement:** Prove the following by contraposition. If \( n^2 \) is not divisible by 3, then \( n \) is not divisible by 3. **Explanation:** To prove this statement using contraposition, we demonstrate that the contrapositive is true. The contrapositive of the given statement is: **If \( n \) is divisible by 3, then \( n^2 \) is divisible by 3.** **Proof:** 1. Assume that \( n \) is divisible by 3. This means there exists an integer \( k \) such that \( n = 3k \). 2. Calculate \( n^2 \): \[ n^2 = (3k)^2 = 9k^2 = 3(3k^2) \] 3. Since \( n^2 = 3(3k^2) \), \( n^2 \) is also divisible by 3. Thus, we have shown that if \( n \) is divisible by 3, then \( n^2 \) is divisible by 3, proving the contrapositive and therefore the original statement.
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