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Sums Of Infinte Series

  1. Mar 16, 2009 #1
    Decide (with justification) if the following series converges or diverges;

    Sum(1,infinty) (2^(n)+3^(n))/(4^(n)+5^(n))

    I've tried using the ratio test but I couldn't see that it was helping in any way, should I be using a different type of test for this problem? I really can't see where to start with this one.
     
  2. jcsd
  3. Mar 16, 2009 #2

    Dick

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    Try and think of a clever comparison to which you can apply the ratio test. E.g. (2^n+3^n)/(4^n+5^n)<=(3^n+3^n)/(4^n+4^n). See, I substituted a larger numerator and a smaller denominator?
     
  4. Mar 16, 2009 #3
    So if you apply the ratio test to the (3^(n)+3^(n))/(4^(n)+4^(n)) you find that this series converges as l<1 (l=3/4?). Is it then allowable to say that the original series converges as it is less than (3^(n)+3^(n))/(4^(n)+4^(n)) and therefore the limit must be lees than the limit of the above series and hence it must converge.
     
  5. Mar 16, 2009 #4

    Dick

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    You tell me, ok? Look up the comparison test for series and make sure all the requirements are fulfilled. It's good practice.
     
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