26. Show that the series is divergent using the Integral test 1 n=2

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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**Problem 26.** Show that the series is divergent using the Integral Test:

\[
\sum_{n=2}^{\infty} \frac{1}{\sqrt{n}}
\]

**Explanation:**

To determine if the series diverges or converges, apply the Integral Test. The Integral Test states that for a function \( f(n) = \frac{1}{\sqrt{n}} \), which is positive, continuous, and decreasing for \( n \geq 2 \), we can consider the following improper integral for \( x \geq 2 \):

\[
\int_{2}^{\infty} \frac{1}{\sqrt{x}} \, dx
\]

Calculate the integral:

1. Use the substitution \( u = \sqrt{x} \), so \( x = u^2 \) and \( dx = 2u \, du \).
2. Substitute and integrate:

\[
\int \frac{1}{\sqrt{x}} \, dx = \int \frac{1}{u} \cdot 2u \, du = 2\int 1 \, du = 2u = 2\sqrt{x}
\]

3. Evaluate the bounds from 2 to \(\infty\):

\[
\left[ 2\sqrt{x} \right]_{2}^{\infty} = \lim_{b \to \infty} (2\sqrt{b} - 2\sqrt{2})
\]

The limit \(\lim_{b \to \infty} 2\sqrt{b} = \infty\).

Since this integral diverges, by the Integral Test, the series \(\sum_{n=2}^{\infty} \frac{1}{\sqrt{n}}\) is also divergent.
Transcribed Image Text:**Problem 26.** Show that the series is divergent using the Integral Test: \[ \sum_{n=2}^{\infty} \frac{1}{\sqrt{n}} \] **Explanation:** To determine if the series diverges or converges, apply the Integral Test. The Integral Test states that for a function \( f(n) = \frac{1}{\sqrt{n}} \), which is positive, continuous, and decreasing for \( n \geq 2 \), we can consider the following improper integral for \( x \geq 2 \): \[ \int_{2}^{\infty} \frac{1}{\sqrt{x}} \, dx \] Calculate the integral: 1. Use the substitution \( u = \sqrt{x} \), so \( x = u^2 \) and \( dx = 2u \, du \). 2. Substitute and integrate: \[ \int \frac{1}{\sqrt{x}} \, dx = \int \frac{1}{u} \cdot 2u \, du = 2\int 1 \, du = 2u = 2\sqrt{x} \] 3. Evaluate the bounds from 2 to \(\infty\): \[ \left[ 2\sqrt{x} \right]_{2}^{\infty} = \lim_{b \to \infty} (2\sqrt{b} - 2\sqrt{2}) \] The limit \(\lim_{b \to \infty} 2\sqrt{b} = \infty\). Since this integral diverges, by the Integral Test, the series \(\sum_{n=2}^{\infty} \frac{1}{\sqrt{n}}\) is also divergent.
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