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Homework Help: Summation problem

  1. Jan 6, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the sum to n terms of the series whose rth term is 3r - 2r

    2. Relevant equations

    3. The attempt at a solution

    So I tried this

    rlog3 - rlog2 = n(n+1)/2 log3 - n(n+1)/2 log2

    then I realized this was kind of useless, the only thing I could get from this is

    n(n+1)/2 log (3/2) , then I don't know what to do with it.

    the answer in the book is 3n+1/2 -2n+1 + 1/2
  2. jcsd
  3. Jan 6, 2014 #2


    Staff: Mentor

    You have a mistake right off the bat. log(A - B) ≠ logA - logB.
    Split your series into two - each part is a geometric series.
  4. Jan 6, 2014 #3
    I see it now thanks.. I keep making those darned mistakes with logs!!
  5. Jan 6, 2014 #4


    Staff: Mentor

    It would be worth your while to review the properties of logs. They might be listed in an appendix of the book you're using. If not, get a precalc book or put in some time in an online tutorial such as khanacademy or the like.
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