Sum of a Power Series: Finding the Sum of a Series with a Variable

In summary, the conversation is about finding the sum of the series \sum_{n=1}^\infty nx^{n-1} , |x|<1 , with the given homework equations \frac{a}{1-r} and \frac{1}{1-x}. The student initially provides a function representation for the series but is unsure how to find the sum with a variable included. The expert asks if the student knows what the series \sum_{n=0}^{+\infty} x^n is, to which the student replies \frac{1}{1-x}. The expert then suggests taking derivatives, which leads the student to realize that their initial function representation was incorrect due to not using the chain rule
  • #1
toothpaste666
516
20

Homework Statement



find the sum of the following series:

[itex] \sum_{n=1}^\infty nx^{n-1} , |x|<1 [/itex]

Homework Equations



[itex] \frac{a}{1-r} [/itex]

The Attempt at a Solution



i know that a function representation for that series is [itex] -\frac{1}{(1-x)^2} [/itex] but how is it possible to find the sum of a series with a variable in it? please help :(
 
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  • #2
Do you know what

[tex]\sum_{n=0}^{+\infty} x^n[/tex]

is?
 
  • #3
[itex] \frac{1}{1-x} [/itex]
 
  • #4
Now take derivatives.
 
  • #5
i know that the series as a function is [itex] \frac{-1}{(1-x)^2} [/itex] but webassign said that was wrong. they are looking for the sum of the series.
 
  • #6
toothpaste666 said:
i know that the series as a function is [itex] \frac{-1}{(1-x)^2} [/itex] but webassign said that was wrong. they are looking for the sum of the series.

Yes, it is wrong. Please show your work.
 
  • #7
I wrote it as
[itex] (1-x)^{-1 }[/itex]
to take the derivative i multiplied it by the exponent and subtracted one from the exponent.
[itex] -1(1-x)^{-2} [/itex]
which is
[itex] -\frac{1}{(1-x)^2} [/itex]
 
  • #8
oh wait i see it now. i forgot to use the chain rule. it should be
[itex] \frac{1}{(1-x)^2} [/itex]
 
  • #9
toothpaste666 said:
oh wait i see it now. i forgot to use the chain rule. it should be
[itex] \frac{1}{(1-x)^2} [/itex]

Indeed!
 
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Likes 1 person
  • #10
thanks!
 

1. What is a power series?

A power series is an infinite series of the form ∑n=0∞ an(x-c)n, where an represents a sequence of coefficients and c represents a constant. It is a mathematical expression that represents a function as an infinite sum of terms.

2. What is the sum of a power series?

The sum of a power series is the value obtained when all the terms of the series are added together. It is important to note that the sum of a power series may converge to a finite value or may diverge to infinity.

3. How is the sum of a power series calculated?

The sum of a power series can be calculated using various methods such as the ratio test, the root test, or the comparison test. These methods help determine whether the series converges or diverges and can provide an approximation for the sum.

4. What is the significance of the sum of a power series?

The sum of a power series is significant because it allows us to represent functions as an infinite sum of simpler terms. This can be useful in various areas of mathematics, such as calculus and differential equations, as it provides a way to approximate and manipulate functions.

5. Can all functions be represented as a power series?

No, not all functions can be represented as a power series. A function can only be represented as a power series if it is analytic, meaning it can be represented as a power series at every point within its domain. Some functions, such as trigonometric functions, have infinite power series representations, while others, such as logarithmic functions, have a limited number of terms in their power series representation.

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