Question about the derivative of this sum and where n starts

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    Derivative Sum
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Discussion Overview

The discussion revolves around the differentiation of the power series representation of the function 1/(1-x) and the implications of the starting index for the summation in both the original and derived series. It touches on mathematical reasoning and notation related to series and derivatives.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant questions why the original sum starts at n=0 while the derived sum starts at n=1.
  • Another participant suggests that the second series could also start at n=0 without affecting the result, as the term for n=0 would contribute zero to the sum.
  • A different participant proposes to expand the sum, take the derivative, and then rewrite the result in sum notation, implying a method to clarify the differentiation process.
  • Another participant notes that both series start with the number 1, as substituting n=0 in the first series yields x^0=1.

Areas of Agreement / Disagreement

Participants express differing views on the implications of starting indices for the series, with some suggesting flexibility in notation while others emphasize the potential complications introduced by including n=0 in the derived series. The discussion remains unresolved regarding the best approach to represent the series after differentiation.

Contextual Notes

There are limitations in the discussion regarding the treatment of the term for n=0 in the derived series and the implications for the function at x=0, which some participants note could lead to complications.

Frankenstein19
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Ok so when differentiating

1/(1-x)= Σ xn from n=0 to infinity

the book says it is 1/(1-x)^2 = Σ n*(x)n-1 from n=1 to infinity

i don't understand why the original sum starts at 0 and then the derived sum starts at 1
 
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Both start with the number 1. If you like you may start the second series at 0, too. It doesn't make a difference because ##0 \cdot x^{-1} = 0.## However, in this case one unnecessarily gets into trouble with ##x=0.##
 
Expand the sum and take the derivative and then rewrite the result in sum notation.
 
both starts with 1 because when you put ##n=0## in first series,the answer will be ##x^0=1##
 

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