Power Series Help: Find Interval of Convergence

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Homework Help Overview

The discussion revolves around finding the interval and radius of convergence for a specific infinite series involving a power series. The series in question is expressed as ((-3)^n * x^n) / (n*(n)^(1/2)), and participants are exploring the application of the ratio test to determine convergence properties.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the use of the ratio test and share their attempts at simplification. There is a focus on the behavior of the ratio as n approaches infinity, with one participant suggesting the use of elementary algebra to analyze the limit of n/(n+1).

Discussion Status

The discussion is ongoing, with participants providing insights and questioning the convergence at specific points, such as x = -1/3. While some calculations have been presented, there is no explicit consensus on the final conclusions regarding the interval of convergence.

Contextual Notes

Participants are navigating the complexities of convergence criteria and are considering the implications of their findings on specific boundary values. There is an acknowledgment of potential confusion regarding the application of L'Hopital's rule and the limits involved in the ratio test.

STJ
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Homework Statement


Find interval of convergence and radius of convergence of the following infinite series.

Series from n=1 to infinity ((-3)^n * x^n) / (n*(n)^1/2)

Homework Equations


Ratio test

The Attempt at a Solution



I've started with the ratio test and end up getting 3xn^(3/2) / (n+1)^(3/2) after cancellation. I don't know how to cancel anything else out, I'm guessing L'Hopital's rule but that doesn't seem right. I feel like I should be able to do more cancellation here.
 
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STJ said:

Homework Statement


Find interval of convergence and radius of convergence of the following infinite series.

Series from n=1 to infinity ((-3)^n * x^n) / (n*(n)^1/2)


Homework Equations


Ratio test


The Attempt at a Solution



I've started with the ratio test and end up getting 3xn^(3/2) / (n+1)^(3/2) after cancellation. I don't know how to cancel anything else out, I'm guessing L'Hopital's rule but that doesn't seem right. I feel like I should be able to do more cancellation here.

Just use elementary algebra:
[tex]\frac{n^{3/2}}{(n+1)^{3/2}} = \left( \frac{n}{n+1}\right)^{3/2}[/tex]
What happens to this ratio when ##n \to \infty?##
 
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What can you say about n/(n+1) as n →∞?
 
I swear I think to hard sometimes. Thanks.

And as n/(n+1) n →∞ = 1

R=1/3, Interval of convergence will be [-1/3, 1/3]
 
Are you sure it converges for x = -1/3?
 

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