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Homework Help: Power series representation

  1. Mar 7, 2008 #1
    1. The problem statement, all variables and given/known data
    Use differentiation to find a power series representation for f(x) = 1/ (1+x)^2

    2. Relevant equations
    geometric series sum = 1/(1+x)

    3. The attempt at a solution
    (1) I see that the function they gave is the derivative of 1/(1+x).
    (2) Therefore, (-1)*(d/dx)summation(x^n) = -1/(1+x)^2
    (3) Differentiating the summation gives:
    (-1)*[summation (n)x^(n-1)]

    However, the book is telling me that for my second step (2) I should be getting
    d/dx [summation (-1)^n (x^n)].

    Why is it becoming an alternating series here?
  2. jcsd
  3. Mar 7, 2008 #2


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    Homework Helper


    [tex] \frac{1}{1-x} = 1+x+x^2+.... [/tex]

    so that, after substituting -x for x:

    [tex] \frac{1}{1+x} = 1 - x + x^2 + ... [/tex]

    You can remember the denominator in the first equation is 1-x by multiplying both sides by (1-x), giving:

    [tex] 1 = (1-x)(1+x+x^2+...) = 1 + x + x^2 + ... - x - x^2 -x^3 - ... = 1[/tex]

    which is consistent, as opposed to what you'd get if you assume 1+x+... was 1/(1+x). (By the way, these manipulations of infinite sums aren't strictly valid, but they can be made more rigorous by restricting to finite sums and taking a limit at the end).
    Last edited: Mar 7, 2008
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