,(-1)" converges conditionally, but not absolutely. Show all work. 3n Prove E

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ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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**Homework Problem on Series Convergence**

**Problem**

**a.** Prove \( \sum_{n=1}^{\infty} \frac{(-1)^n}{3n} \) converges conditionally, but not absolutely. Show all work.

**b.** What value of \( n \) must you use to compute the \(n^{th}\) partial sum, \( S_n \), so that \( S_n \) is accurate to within 0.005? You do NOT need to calculate \( S_n \); just tell me what \( n \) works. Show all work.

**Solution Steps**

### Part a: Conditional Convergence Proof

1. **Test for Absolute Convergence:**
   Consider the absolute value of the terms of the series:
   \[
   \sum_{n=1}^{\infty} \left| \frac{(-1)^n}{3n} \right| = \sum_{n=1}^{\infty} \frac{1}{3n}
   \]
   This is a constant multiple of the harmonic series:
   \[
   \sum_{n=1}^{\infty} \frac{1}{n}
   \]
   Since the harmonic series diverges, \(\sum_{n=1}^{\infty} \frac{1}{3n} \) also diverges. Therefore, the series does not converge absolutely.

2. **Test for Conditional Convergence:**
   Use the Alternating Series Test (Leibniz's Test) for the series \( \sum_{n=1}^{\infty} \frac{(-1)^n}{3n} \).

   The Alternating Series Test requires checking two conditions:
   - The terms \( a_n = \frac{1}{3n} \) decrease monotonically.
   - \(\lim_{n \to \infty} a_n = 0 \)

   Both conditions are satisfied:
   - \( a_{n+1} = \frac{1}{3(n+1)} < \frac{1}{3n} = a_n \) for all \( n \), so the terms decrease monotonically.
   - \(\lim_{n \to \infty} \frac{1}{3n} = 0\)

   Thus, by the Alternating Series Test, the series \( \sum_{n=1
Transcribed Image Text:**Homework Problem on Series Convergence** **Problem** **a.** Prove \( \sum_{n=1}^{\infty} \frac{(-1)^n}{3n} \) converges conditionally, but not absolutely. Show all work. **b.** What value of \( n \) must you use to compute the \(n^{th}\) partial sum, \( S_n \), so that \( S_n \) is accurate to within 0.005? You do NOT need to calculate \( S_n \); just tell me what \( n \) works. Show all work. **Solution Steps** ### Part a: Conditional Convergence Proof 1. **Test for Absolute Convergence:** Consider the absolute value of the terms of the series: \[ \sum_{n=1}^{\infty} \left| \frac{(-1)^n}{3n} \right| = \sum_{n=1}^{\infty} \frac{1}{3n} \] This is a constant multiple of the harmonic series: \[ \sum_{n=1}^{\infty} \frac{1}{n} \] Since the harmonic series diverges, \(\sum_{n=1}^{\infty} \frac{1}{3n} \) also diverges. Therefore, the series does not converge absolutely. 2. **Test for Conditional Convergence:** Use the Alternating Series Test (Leibniz's Test) for the series \( \sum_{n=1}^{\infty} \frac{(-1)^n}{3n} \). The Alternating Series Test requires checking two conditions: - The terms \( a_n = \frac{1}{3n} \) decrease monotonically. - \(\lim_{n \to \infty} a_n = 0 \) Both conditions are satisfied: - \( a_{n+1} = \frac{1}{3(n+1)} < \frac{1}{3n} = a_n \) for all \( n \), so the terms decrease monotonically. - \(\lim_{n \to \infty} \frac{1}{3n} = 0\) Thus, by the Alternating Series Test, the series \( \sum_{n=1
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