• Support PF! Buy your school textbooks, materials and every day products Here!

Fourier series question

  • Thread starter thomas49th
  • Start date
  • #1
655
0

Homework Statement


A 2pi peroidic function f is defined in the interval (-pi, pi) by f = t. Sketch the graph of the fucntion and show that it's Fourier series is given by

[tex]\frac{\pi^{2}}{3} + 4\sum^{\infty}_{n=1}\frac{(-1)^{n} \cos(nt)}{n^{2}}[/tex]



Homework Equations




The Attempt at a Solution




Well if you draw the function you can see that it's odd therefore [tex]a_{n} = 0 a_{0} = 0[/tex]

[tex]b_{n} = \frac{1}{\pi} \int_{-\pi}^{\pi} t\sin(nt)dt[/tex]

This has to be done by parts

giving [tex]\frac{1}{\pi}[\frac{t-\cos(nt)}{n} + \frac{1}{n}\int \cos(nt) dt[/tex]

We can ignore the latter term as our limits are pi and the integral of cosine is sine and sine of any multiple of pi is 0. This means

[tex]b_{n} = \frac{1}{n\pi}[t.-\cos(nt)]^{\pi}_{-\pi}[/tex]

After plugging in the limits I find this to eqal -2pi cos(npi) which is -2pi(-1)^n

Not what it's meant to equal :(

I don't see where the n^2 comes from in the original question nor the DC value. Actually I dont see where there answer comes from altogether.

Any suggestions

Thanks
Thomas
 
Last edited:

Answers and Replies

  • #2
655
0
apparently the question set was incorrect... After being given the correct question I solved it (f(t) = t^2)
 

Related Threads on Fourier series question

  • Last Post
Replies
6
Views
3K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
2
Views
944
  • Last Post
Replies
3
Views
826
  • Last Post
Replies
2
Views
841
  • Last Post
Replies
0
Views
1K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
2
Views
927
  • Last Post
Replies
0
Views
1K
Top