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Using Fourier Series

  1. Jul 14, 2007 #1

    I am having trouble understanding how to use Fourier series. To be more specific, here's what I mean.


    My question about those formulas is, how do I know what f(t) is?
    When I do excercises, I never get what f(t) is. Can anyone tell me how to find it?
  2. jcsd
  3. Jul 14, 2007 #2


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    Isn't f(t) the function for which you are attempting to find a fourier series expansion?
  4. Jul 14, 2007 #3
    Yes, but in excersies that I've tried doing, I am not told f(t). I only get a graph usually.
  5. Jul 14, 2007 #4


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    Can you not spot an equation for the graph? Why don't you post an example, and it'll be easier to help.
  6. Jul 14, 2007 #5
    Yes, here is one of the excersises. I need to find the fourier series of that function:

  7. Jul 14, 2007 #6


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    Use the graph to define your f(t). This called a piecewise function, it means you will need to break the integrals into pieces which correspond to the different parts of the function.

    0 <= t < 50 f(t) = 40
    50 <= t < 100 f(t) = 80
    100 <= t < 150 f(t) = -40
    150<= t <200 f(t) = -80

    Now simply evaluate the integrals, using the different segments as that limits for each section.
  8. Jul 14, 2007 #7
    Alright, so I took your advice, but my results are still wrong. Here's what I did:

    from 0 to 50, and I got 40, but the answer is 0.

    EDIT: The answer I got was correct, I just didn't finish. Thank you cristo and Integral for your help.
    Last edited: Jul 14, 2007
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