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Homework Help: Radius of convergence log(a + x)

  1. Apr 25, 2014 #1
    1. The problem statement, all variables and given/known data

    determine the radius of convergence of the series expansion of log(a + x) around x = 0

    2. Relevant equations

    3. The attempt at a solution

    So after applying the Taylor series expansion about x=0 we get log(a) + SUM[(-1)^n x^n/(n a^n)] I understand how to get the radius of convergence for log(1+x) for instance using the ratio test, but with 1 being replaced with what I believe to be an arbitrary constant I am now confused as to how I should tackle this, any help would be great, thanks in advance!

    Would I employ the ratio test in the same way? I have a log in the numerator and denominator so not really sure...
  2. jcsd
  3. Apr 25, 2014 #2


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    Homework Helper

    There is a minor error: [tex]log(a+x)=log(a)+\sum_1^{\infty}(-1)^{n+1} \frac{x^n}{n a^n}[/tex]
    Do the same as with other power series.
    Where do you have log in the terms of the series, except the first one?

    Last edited: Apr 25, 2014
  4. Apr 25, 2014 #3
    Yeah so I only have the log in the first term, that's why I am confused, just making sure it is the same as if all terms are just a power series, would I use the ratio test in the same vein as with log(1+x)?
  5. Apr 25, 2014 #4


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    Homework Helper

    Yes, use the ratio test. Do not worry about the first term.

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