5. For all positive integer n, n³ +5n+6 is divis We use induction on n Then (n+12²³+ 5(n+1) 76 = n²³² +31²³²2w 135 (n+1) (ht? 1²72+1 (1)

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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Use induction method

**Mathematical Induction Problem:**

5. **Problem Statement**
   - For all positive integers \( n \), prove that \( n^3 + 5n + 6 \) is divisible by 3.

**Solution Approach:**

- **Induction Hypothesis:** Assume for an integer \( n \), the expression \( n^3 + 5n + 6 \) is divisible by 3.

- **Basis Step:** Verify for \( n = 1 \):
  \[
  1^3 + 5 \times 1 + 6 = 1 + 5 + 6 = 12
  \]
  12 is divisible by 3.

- **Inductive Step:** Assume true for \( n = k \), i.e., \( k^3 + 5k + 6 \) is divisible by 3.
  - Show that it holds for \( n = k + 1 \):
  \[
  (k+1)^3 + 5(k+1) + 6 = k^3 + 3k^2 + 3k + 1 + 5k + 5 + 6
  \]
  \[
  = k^3 + 5k + 6 + 3k^2 + 3k + 12
  \]
  The expression can be rearranged to show divisibility by 3.

Thus, by induction, \( n^3 + 5n + 6 \) is divisible by 3 for all positive integers \( n \).

**Conclusion:**
The problem demonstrates a typical use of mathematical induction to prove divisibility properties for polynomial expressions dependent on integer variables.
Transcribed Image Text:**Mathematical Induction Problem:** 5. **Problem Statement** - For all positive integers \( n \), prove that \( n^3 + 5n + 6 \) is divisible by 3. **Solution Approach:** - **Induction Hypothesis:** Assume for an integer \( n \), the expression \( n^3 + 5n + 6 \) is divisible by 3. - **Basis Step:** Verify for \( n = 1 \): \[ 1^3 + 5 \times 1 + 6 = 1 + 5 + 6 = 12 \] 12 is divisible by 3. - **Inductive Step:** Assume true for \( n = k \), i.e., \( k^3 + 5k + 6 \) is divisible by 3. - Show that it holds for \( n = k + 1 \): \[ (k+1)^3 + 5(k+1) + 6 = k^3 + 3k^2 + 3k + 1 + 5k + 5 + 6 \] \[ = k^3 + 5k + 6 + 3k^2 + 3k + 12 \] The expression can be rearranged to show divisibility by 3. Thus, by induction, \( n^3 + 5n + 6 \) is divisible by 3 for all positive integers \( n \). **Conclusion:** The problem demonstrates a typical use of mathematical induction to prove divisibility properties for polynomial expressions dependent on integer variables.
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