Finding the Sum of a Power Series: Tips and Tricks for Success

In summary, the conversation is about trying to find the sum of a series involving k^2 and x^(n-1). The attempted solution involves using derivatives and breaking the series down into smaller parts. The final solution involves taking the derivative of the function and summing the initial x^(k-1) as a geometric series. The final answer is 700/729, which was initially thought to be incorrect, but was later confirmed to be correct.
  • #1
peripatein
880
0

Homework Statement


I am trying to find the sum of the series in the attachment.


Homework Equations





The Attempt at a Solution


I have tried to use various series and their derivatives, to not much avail.
I am not sure how to handle the n^2 factor.
Should I break it down to two series?
Any suggestions?
 

Attachments

  • Sum.jpg
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  • #2
You know about derivatives? Good do this

[tex]\sum_{k=1}^\infty k^2 x^{n-1}=\left( x \left( x \sum_{k=1}^\infty x^{k-1}\right)^\prime \right)^\prime=\left( x \left( x \frac{1}{1-x}\right)^\prime \right)^\prime[/tex]

|x|<1
your case will be x=1/10
 
  • #3
If I am not mistaken, this yields 700/729, which, according to Wolfram, is incorrect. Would you please account for that?
 
  • #4
peripatein said:
If I am not mistaken, this yields 700/729, which, according to Wolfram, is incorrect. Would you please account for that?

You are mistaken. Check it again.
 
  • #5
Would you please explain how it was arrived at?
 
  • #6
peripatein said:
Would you please explain how it was arrived at?

Basically you take x^(k-1). Multiplying by x and differentiating gives you k*x^(k-1). Doing the same thing again gives k^2*x^(k-1). Which is the form you want. Now sum the initial x^(k-1) as a geometric series and repeat the same sequence of operations on the function of you get.
 

Related to Finding the Sum of a Power Series: Tips and Tricks for Success

1. What is a power series?

A power series is an infinite series of the form ∑(n=0)∞anxn, where an represents the coefficients and x represents the variable. It is a type of mathematical series that is used to represent a function as a sum of infinitely many terms.

2. How do you find the sum of a power series?

The sum of a power series can be found by using the formula ∑(n=0)∞anxn = a0 + a1x + a2x2 + a3x3 +..., where the value of x is substituted into the equation to calculate the sum. The series must converge in order to find the sum.

3. What is the difference between a finite and infinite power series?

A finite power series has a limited number of terms, while an infinite power series has an infinite number of terms. A finite power series can be evaluated to a specific value, while an infinite power series can only be approximated through partial sums.

4. How do you determine if a power series converges or diverges?

A power series converges if the limit of its terms approaches zero as n approaches infinity, meaning that the series approaches a finite value. It diverges if the limit does not approach zero, meaning that the series does not have a finite sum.

5. What is the significance of the radius of convergence in a power series?

The radius of convergence is the distance from the center of the power series to the nearest point where the series converges. It is important because it determines the values of x for which the series will converge, and it can also be used to determine the interval of convergence for the series.

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