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Homework Help: Maclaurin Series using Substitution

  1. Apr 17, 2014 #1
    1. The problem statement, all variables and given/known data
    Use a known Maclaurin series to compute the Maclaurin series for the function: f(x) = x/(1-4(x^2))

    2. Relevant equations
    1/(1-x) = ∑x^n

    3. The attempt at a solution
    I tried removing x from the numerator for: x ∑ 1/(1-4(x^2)), which would end up through substitution as x ∑ (4^n)(x^2n). Not too sure this is correct use of substitution however.
  2. jcsd
  3. Apr 17, 2014 #2
    Yes, it is correct.
  4. Apr 17, 2014 #3


    Staff: Mentor

    What you did makes sense, but how you described what you did doesn't make sense. If you can find the series for 1/(1 - 4x2), just multiply term-by-term to get the series for x/(1 - 4x2). Pulling a variable out of a summation that involves x isn't a valid operation.
    For example,
    $$\sum_{n = 1}^k n^2 \neq n \cdot \sum_{n = 1}^k n $$
  5. Apr 18, 2014 #4
    So in essence, I would write the terms of the series ∑ (4^n)(x^2n) and multiply each term by x? Alright, is there no way of writing the series so that there is no x variable outside of the summation?
  6. Apr 18, 2014 #5


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Mark did say to take the sum, then multiply through by x, term by term. (Basically, that's the distributive law, and the extra x will be inside of the sum.

    For example,

    ##\displaystyle x\left(\sum _{n=1}^ k x^n \right) = \sum _{n=1}^ k x^{n+1} ##
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