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Homework Help: Find the interval of convergence for the given power series.

  1. Apr 29, 2012 #1
    1. The problem statement, all variables and given/known data

    Find the interval of convergence for the given power series.
    Sum from n=1 to infinty of (x-11)^n / (n(-9)^n)

    2. Relevant equations

    3. The attempt at a solution
    I used the ratio test and I'm getting 2<x<20, but that doesn't seem to be right. I get abs(1/9*(x-11)) < 1, which simplifies into 2<x<20. I couldn't sworn I did this problem correctly, but I'm not getting the correct answer. Any help would be great.
  2. jcsd
  3. Apr 29, 2012 #2


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    Science Advisor
    Homework Helper
    Gold Member

    That is the correct interval, but don't forget you have to check the two end points separately.
  4. Apr 29, 2012 #3
    Here the thing, I entered all possible compinations for the endpoints just to check my answer for the interval and they were all wrong. Just to double check it does not converge at 2 and does at 20, right?
  5. Apr 29, 2012 #4
    Ok it accepts my answer now. It was probably something on my end but thanks anyways.
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