Write S = Σ n=5 SN = S = 1 n(n-1) as a telescoping series and find its sum.

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Chapter1: Functions And Models
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**Problem:**

Write \( S = \sum_{n=5}^{\infty} \frac{1}{n(n-1)} \) as a telescoping series and find its sum.

**Solution Steps:**

1. **Identify the Series:**
   The given series is \( S = \sum_{n=5}^{\infty} \frac{1}{n(n-1)} \).

2. **Decompose into Partial Fractions:**
   The goal is to rewrite \( \frac{1}{n(n-1)} \) as a difference of two fractions:
   \[
   \frac{1}{n(n-1)} = \frac{1}{n-1} - \frac{1}{n}
   \]

3. **Form the Telescoping Series:**
   Substitute back into the series:
   \[
   S = \sum_{n=5}^{\infty} \left( \frac{1}{n-1} - \frac{1}{n} \right)
   \]
   This type of series allows terms to cancel out sequentially (telescoping effect).

4. **Calculate the Partial Sum \( S_N \):**
   The partial sum \( S_N \), from \( n=5 \) to \( n=N \), is
   \[
   S_N = \left( \frac{1}{4} - \frac{1}{5} \right) + \left( \frac{1}{5} - \frac{1}{6} \right) + \ldots + \left( \frac{1}{N-1} - \frac{1}{N} \right)
   \]

5. **Show Cancellation:**
   Observe that this series telescopes such that most terms cancel:
   \[
   S_N = \frac{1}{4} - \frac{1}{N}
   \]

6. **Calculate the Infinite Sum \( S \):**
   As \( N \to \infty \), \( \frac{1}{N} \to 0 \), so the sum of the infinite series is:
   \[
   S = \frac{1}{4}
   \]

**Solutions:**
- \( S_N = \frac{1}{4} - \frac{1}{N} \)
- \( S = \frac{1}{4} \)
Transcribed Image Text:**Problem:** Write \( S = \sum_{n=5}^{\infty} \frac{1}{n(n-1)} \) as a telescoping series and find its sum. **Solution Steps:** 1. **Identify the Series:** The given series is \( S = \sum_{n=5}^{\infty} \frac{1}{n(n-1)} \). 2. **Decompose into Partial Fractions:** The goal is to rewrite \( \frac{1}{n(n-1)} \) as a difference of two fractions: \[ \frac{1}{n(n-1)} = \frac{1}{n-1} - \frac{1}{n} \] 3. **Form the Telescoping Series:** Substitute back into the series: \[ S = \sum_{n=5}^{\infty} \left( \frac{1}{n-1} - \frac{1}{n} \right) \] This type of series allows terms to cancel out sequentially (telescoping effect). 4. **Calculate the Partial Sum \( S_N \):** The partial sum \( S_N \), from \( n=5 \) to \( n=N \), is \[ S_N = \left( \frac{1}{4} - \frac{1}{5} \right) + \left( \frac{1}{5} - \frac{1}{6} \right) + \ldots + \left( \frac{1}{N-1} - \frac{1}{N} \right) \] 5. **Show Cancellation:** Observe that this series telescopes such that most terms cancel: \[ S_N = \frac{1}{4} - \frac{1}{N} \] 6. **Calculate the Infinite Sum \( S \):** As \( N \to \infty \), \( \frac{1}{N} \to 0 \), so the sum of the infinite series is: \[ S = \frac{1}{4} \] **Solutions:** - \( S_N = \frac{1}{4} - \frac{1}{N} \) - \( S = \frac{1}{4} \)
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