How Can You Find a Closed Form for the Series x + 2x² + 3x³ + 4x⁴ + ...?

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Homework Help Overview

The discussion revolves around finding a closed form for the series x + 2x² + 3x³ + 4x⁴ + ..., which can be expressed as the sum ∑(n=1 to ∞) nxⁿ. Participants are exploring methods to derive a closed formula for this series.

Discussion Character

  • Exploratory, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants suggest differentiating the geometric series formula as a potential approach. Others mention combining the original series with a modified version (xS) to simplify the problem. There is also a reference to the series being an arithmetic geometric progression (AGP) and the idea of manipulating the series to derive a simpler geometric progression.

Discussion Status

The discussion is active, with multiple approaches being considered. Participants are sharing insights and methods without reaching a consensus on a single solution. The exploration of different techniques indicates a productive direction in understanding the series.

Contextual Notes

There is an underlying assumption that participants are familiar with the properties of geometric series and their derivatives. The discussion does not provide complete information on the convergence criteria for the series in question.

noblerare
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Homework Statement



I want to find a closed form formula for:

[tex]x+2x^2+3x^3+4x^4+\ldots[/tex]

I know that this can be written as:

[tex]\sum_{n=1}^{\infty}nx^n[/tex]

but I would like to have a closed formula for this.

The formula for an infinite geometric series is:
[tex]\sum_{n=0}^{\infty}x^n = \frac{1}{1-x}[/tex]

Which is somewhat close but the series in question is not exactly geometric.

How do I go about doing this?
 
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Try differentiating the formula for the geometric series.
 
Also, the usual "trick" for deriving geometric series also works for that one -- combine the original series S with the series xS to produce something simpler.
 
this sort of series is called an arithmetic geometric progression (AGP) or something...like someone said, multiply by x and then subtract to get a simple geometric progression...in this case you could even divide by x
 

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