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Homework Help: Power Series Representation of (1+x)/(1-x)

  1. Apr 16, 2013 #1
    1. The problem statement, all variables and given/known data

    For the power series representation of, f(x)=1+x1−x which is 1+2∑from n=1 to inf (x^n), Where does the added 1 in front come from? How do I get to this answer from ∑n=0 to inf (x^n)+∑n=0 to inf (x^(n+1))

    2. Relevant equations

    3. The attempt at a solution

    I arrived at ∑n=0 to inf x^n + ∑ n=0 to inf x^(n+1)
  2. jcsd
  3. Apr 16, 2013 #2
    The 1 in front comes from using long division to isolate the 1/(1-x) term. I'm not sure what you're asking in your second question though. If you want to change the index of a summation, you can do it entirely artificially. Namely, if you want [itex] \sum_{n=1}^\infty x^n [/itex] to look like a sum where the lower index starts at 0 instead of 1, define a new index k = n-1.
  4. Apr 17, 2013 #3


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  5. Apr 17, 2013 #4
    Rereading your question, I now understand what you are saying. Have you been doing as suggested? Use long division to break up the rational function, then use vela's comment about how [itex] x^0 = 1 [/itex] and you'll get the answer your originally posted.
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