How Do You Simplify and Find the Interval of Convergence for a Power Series?

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SUMMARY

The discussion focuses on finding the interval of convergence for the power series \(\sum_{n=1}^{\infty} \frac{x^n}{n \sqrt{n} 3^n}\) using the Ratio Test. The user successfully simplifies the limit expression to \(\lim_{n \to \infty} \frac{|x|}{3} \cdot (1/(1+1/n))^{3/2}\), which ultimately leads to the conclusion that the interval of convergence is determined by \(|x| < 3\). The Ratio Test is confirmed as the appropriate method for this analysis.

PREREQUISITES
  • Understanding of power series and their convergence
  • Familiarity with the Ratio Test for series convergence
  • Basic knowledge of limits in calculus
  • Ability to manipulate algebraic expressions involving limits
NEXT STEPS
  • Study the application of the Ratio Test in different types of series
  • Explore the concept of absolute convergence in power series
  • Learn about other convergence tests, such as the Root Test
  • Investigate the behavior of power series at the endpoints of their interval of convergence
USEFUL FOR

Students and educators in calculus, mathematicians focusing on series convergence, and anyone seeking to deepen their understanding of power series and convergence tests.

tolove
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Edit: Nevermind, figured it out. Thank you for readingOriginal problem:
Find the interval of convergence
[itex]\sum[/itex]n=1 xn / n * √(n) * 3n

Ratio Test, right? an+1/a

I get to here and I can't figure out how to get rid of the ns:

lim n→∞ abs(x/3)* [n*√(n) / (n+1)*√(n+1)]

Solution,
They break apart evenly:
(n/(n+1)) * (n/(n+1)**(1/2)

(also, sorry this looks terrible. I'm not sure how to use the graphics options very well yet)
 
Last edited:
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tolove said:
Original problem:
Find the interval of convergence
[itex]\sum[/itex]n=1 xn / n * √(n) * 3n

Ratio Test, right? an+1/a

I get to here and I can't figure out how to get rid of the ns:

lim n→∞ abs(x/3)* [n*√(n) / (n+1)*√(n+1)]

(also, sorry this looks terrible. I'm not sure how to use the graphics options very well yet)

Start with just the n/(n+1) part. Divide numerator and denominator by n and tell me what the limit of that is as n->infinity.
 
Dick said:
Start with just the n/(n+1) part. Divide numerator and denominator by n and tell me what the limit of that is as n->infinity.

Simplifies down to:
limn→∞ abs(x)/3 * (1/(1+1/n))**(3/2) = abs(x)/3

Thank you very much for your reply!
 

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