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Homework Help: Fourier series How to integrate SinxCosnx?

  1. May 5, 2009 #1
    1. The problem statement, all variables and given/known data
    Compute the fourier cosine series for given function:

    f(x)=sinx 0<x<pi

    2. Relevant equations

    for cosine series of f(x) on [0,T]... use this general equation:

    3. The attempt at a solution

    so I get:

    a0 = (2/pi) * integral(sinxdx) with bounds 0 to pi = 4/pi

    but then.. when I try to compute an

    I get
    an=(2/pi) * integral(sinx*cosnx*dx) with bounds 0 to pi
    How do I integrate sinxcosnx?
    Last edited: May 5, 2009
  2. jcsd
  3. May 5, 2009 #2


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    Homework Helper

    How did you go from f(x)=e^x to f(x)=sin(x)? Either way use the product-to-sum identity to write sin(x)cos(nx) as a sum of sine functions or express the sine and cosine functions in terms of complex exponentials.
  4. May 5, 2009 #3
    Ah, sorry I'm becoming delusional from doing too much work in one day. It's fixed now.

    I do get sin(x)cos(nx) right? Or am I doing something wrong
  5. May 5, 2009 #4


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    Homework Helper

    Yes you do get sin(x)cos(nx) with the edited f(x). Now use [tex]\sin \theta \cos \varphi = {\sin(\theta + \varphi) + \sin(\theta - \varphi) \over 2}[/tex]
  6. May 5, 2009 #5
    Ah, thanks so much. Sorry I actually have another question though,

    I am asked to compute the Fourier series for the following 2 part function:

    f(x)=1 -2<x<0
    f(x)=x 0<x<2

    I'm supposed to do this using the "Euler formulas" not the cos/sin formulas.

    However, I'm not sure how this two part thing works. When trying to find an, do I just do the integral of part 1 + integral of part 2?

    an = [(1/2)*integral(1*cos(n*pi*x/2)) from -2 to 0] + [(1/2)*integral(x*cos(n*pi*x/2)) from 0 to 2]
  7. May 5, 2009 #6


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    Homework Helper

    You're splitting the integral up correctly, but you said you're supposed to use the exponential form of the Fourier-transform after which you use the cosine form instead (confusing). Do it again using the exponential form.
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