Finding the Power Series of f'(x) from f(x) = x^2cos(2x)

In summary, the student is trying to find the power series for f'(x) given f(x) = x2cos2(x), but is having trouble figuring out how to take the derivative of the series equation.
  • #1
Magnawolf
25
0

Homework Statement



Find the power series of f'(x), given f(x) = x2cos2(x)

Homework Equations



attachment.php?attachmentid=58196&stc=1&d=1366887854.png

Correct me if I'm wrong

The Attempt at a Solution



Can I just take the derivative of the solution I got previously? If so, what's a good way to write the sequence out so I can easily make a series representation. Or is there a better approach to the problem?
 

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  • #2
Looks good. I would multiply the ##x^2/2## into the parentheses and include it under the summation. Then differentiate it.
 
  • #3
My main problem is trying to figure out how to represent what I'm getting as a series. When I multiply in the x^2 and differentiate I get

x + x - 8x^3 + 4x^5 - (32/45)x^7...

I can't figure it out
 
  • #4
When you multiply the ##\frac{x^2} 2## in you have$$
\frac {x^2} 2 +\sum_{n=0}^\infty\frac{(-1)^n2^{2n}x^{2n+2}}{(2n)!}$$Just differentiate the first term as you did and differentiate under the sum and you will have your formula.
 
  • #5
When you say differentiate under the sum, do you mean differentiate each term in the sequence or do you mean you can actually take the derivative of the series equation? I tried googling and checking my textbook and I don't know how to take a derivative of a series in equation form, with the n's and such. If you can show me, that'd be great!
 
  • #6
Yes, "differentiate under the sum" means "differentiate term by term". I don't know what you mean by "take the derivative of the equation"!
 
  • #7
Magnawolf said:
When you say differentiate under the sum, do you mean differentiate each term in the sequence or do you mean you can actually take the derivative of the series equation? I tried googling and checking my textbook and I don't know how to take a derivative of a series in equation form, with the n's and such. If you can show me, that'd be great!

If you look at each term inside the sum, it just a constant times a power of ##x##. Use the power rule.
 
  • #8
Oh okay, I got it now. Thanks for everything man!
 

1. What is a power series of f'(x)?

A power series of f'(x) is an infinite series representation of the derivative of a given function f(x). It is written in the form of a sum of terms where each term contains a variable, typically x, raised to a non-negative integer power.

2. How is a power series of f'(x) calculated?

A power series of f'(x) can be calculated using the Taylor series expansion, which involves taking the derivatives of the given function f(x) and evaluating them at a specific point. The coefficients of the resulting series correspond to the coefficients of the power series of f'(x).

3. What is the significance of a power series of f'(x)?

A power series of f'(x) provides a way to approximate the derivative of a function at any point within its radius of convergence. It can also be used to represent a function as a series of simpler functions, making it easier to analyze and manipulate.

4. What is the radius of convergence for a power series of f'(x)?

The radius of convergence for a power series of f'(x) is the distance from the center of the series, where x=0, to the point where the series converges. It is typically determined by using the ratio or root test on the series.

5. Can a power series of f'(x) be used to find the antiderivative of a function?

Yes, a power series of f'(x) can be integrated term by term to find the antiderivative of a function, just like a regular power series. This method is known as the method of undetermined coefficients.

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