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Homework Help: Calculus III series problem

  1. Aug 28, 2009 #1
    I've been working on this for two hours and have had zero luck:


    sum{k=1 to k=oo} [((-1)^(k+1))/k]

    Rearrange the terms so the series converges to 5 [lol, I haven't a clue how].
  2. jcsd
  3. Aug 28, 2009 #2


    Staff: Mentor

    Take a look at this Wikipedia article: http://en.wikipedia.org/wiki/Riemann_series_theorem
    The reason you can use this theorem is that your series is conditionally convergent but not absolutely convergent.

    BTW, here is your series using LaTeX code:
    [tex]\sum_{k = 1}^\infty \frac{(-1)^{k + 1}}{k}[/tex]
  4. Aug 28, 2009 #3


    User Avatar
    Science Advisor

    Separate even (positive) terms as [itex]a_n[/itex] and odd (negative) terms, as [itex]b_n[/itex] Then your series itself is [itex]a_n+ b_n[/itex] while the absolute value is [itex]a_n- b_n[/itex]. You can show the the series involving [itex]a_n[/itex] only goes to infinity while the series involving only [itex]b_n[/itex] goes to negative infinity. Okay, take series only from [itex]a_n[/itex] until the sum is greater than 5. Since that sum minus 5 is a finite number, you add take terms from [itex]b_n[/itex] until that sum is back less than 5. Now add terms from [itex]a_n[/itex] until it is back larger than 5, etc.
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