How do I find the MacLaurin series for $\frac{1}{1 - 2x}$?

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Discussion Overview

The discussion revolves around finding the Maclaurin series for the function $\frac{1}{1 - 2x}$. Participants explore the relationship between this function and the known series for $\frac{1}{1 - x}$, considering convergence and notation.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about directly substituting $2x$ into the known series for $\frac{1}{1 - x}$.
  • Another participant confirms that substituting $2x$ is valid and notes the convergence condition $|x| < \frac{1}{2}$.
  • A participant suggests that the series can be expressed as $\sum_{n = 0}^{\infty} 2^n x^n$ and asks for confirmation of this form.
  • Two participants agree that the notation $\sum_{n=0}^\infty(2x)^n$ is preferred for clarity.
  • One participant emphasizes that since the series is specifically requested as a Maclaurin series, the notation should follow the form $\sum_{n = 0}^{\infty}{c_n\,x^n}$, suggesting that the original notation is acceptable.

Areas of Agreement / Disagreement

There is general agreement on the validity of substituting $2x$ into the series, but there are differing opinions on the preferred notation for expressing the series.

Contextual Notes

The discussion does not resolve the preference for notation, as participants express different views on how to represent the series while adhering to the Maclaurin series format.

tmt1
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I need to find the maclaurin series of the function

$$\frac{1}{1 - 2x}$$.

I know $\frac{1}{1 - x}$ is $1 + x + x^2 + x^3 ...$ but how can I use this to solve the problem? I don't think I can just plug in $2x$ can I?
 
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Yes; plug in $2x$ and note that the series converges for $|x|<\dfrac12$.
 
greg1313 said:
Yes; plug in $2x$ and note that the series converges for $|x|<\dfrac12$.

So, I would get something like

$$\sum_{n = 0}^{\infty} 2^nx^n$$

Is this correct?
 
Yes, but I think the notation

$$\sum_{n=0}^\infty(2x)^n$$

would be preferred. :)
 
greg1313 said:
Yes, but I think the notation

$$\sum_{n=0}^\infty(2x)^n$$

would be preferred. :)

Actually, as it's specifically asked to be written as a MacLaurin Series, where it should be written $\displaystyle \begin{align*} \sum_{n = 0}^{\infty}{c_n\,x^n} \end{align*}$ the OP's notation would be preferred.
 

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