2n+3 22. Lm=1 n+7 Using the Limit Comparison Test show if the series is divergent or convergent

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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To the 4th power
**Problem 22:**

Consider the infinite series 

\[
\sum_{n=1}^{\infty} \frac{2n+3}{n^4+7}
\]

Using the Limit Comparison Test, determine if the series is divergent or convergent.

**Explanation:**

The Limit Comparison Test involves comparing the given series with another series that is known to converge or diverge. The idea is to select a simpler series, usually a p-series or a geometric series, that resembles the given series for large values of \( n \).

In this case, a suitable choice for comparison might be the series

\[
\sum_{n=1}^{\infty} \frac{1}{n^3}
\]

because \(\frac{2n+3}{n^4+7}\) behaves like \(\frac{1}{n^3}\) as \( n \) becomes very large, due to the highest degree terms in the numerator and denominator.

To apply the Limit Comparison Test, compute:

\[
\lim_{n \to \infty} \frac{\frac{2n+3}{n^4+7}}{\frac{1}{n^3}} = \lim_{n \to \infty} \frac{(2n+3) \cdot n^3}{n^4+7}
\]

Simplify the expression and evaluate the limit. If the limit exists and is a positive finite number, then both series will either converge or diverge together. 

Since \(\sum_{n=1}^{\infty} \frac{1}{n^3}\) is a convergent p-series (where \( p = 3 > 1 \)), the original series will also be convergent if the limit is positive and finite.
Transcribed Image Text:**Problem 22:** Consider the infinite series \[ \sum_{n=1}^{\infty} \frac{2n+3}{n^4+7} \] Using the Limit Comparison Test, determine if the series is divergent or convergent. **Explanation:** The Limit Comparison Test involves comparing the given series with another series that is known to converge or diverge. The idea is to select a simpler series, usually a p-series or a geometric series, that resembles the given series for large values of \( n \). In this case, a suitable choice for comparison might be the series \[ \sum_{n=1}^{\infty} \frac{1}{n^3} \] because \(\frac{2n+3}{n^4+7}\) behaves like \(\frac{1}{n^3}\) as \( n \) becomes very large, due to the highest degree terms in the numerator and denominator. To apply the Limit Comparison Test, compute: \[ \lim_{n \to \infty} \frac{\frac{2n+3}{n^4+7}}{\frac{1}{n^3}} = \lim_{n \to \infty} \frac{(2n+3) \cdot n^3}{n^4+7} \] Simplify the expression and evaluate the limit. If the limit exists and is a positive finite number, then both series will either converge or diverge together. Since \(\sum_{n=1}^{\infty} \frac{1}{n^3}\) is a convergent p-series (where \( p = 3 > 1 \)), the original series will also be convergent if the limit is positive and finite.
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