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Find the fourier series of (sinx)^3

  1. Nov 28, 2006 #1
    I'm confused.

    Trying to find the fourier series of (sinx)^3. This is an odd function, so I try to find the fourier sine coefficient, with integral of (sinx)^3*sinkx. However, my answer comes up with all sine terms. Of course all these terms go to zero when integrating between 0 and pi. Am I doing something wrong? Or is there some identity for sinx^3 that makes it easier to solve?
  2. jcsd
  3. Nov 29, 2006 #2


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    If you draw a graph of the function you are trying to integrate, you will see that you are doing something wrong, since the area under the curve isn't zero.

    There are some standard identities that will help. Try expanding sin 3x = sin (2x + x), then sin 2x = sin(x+x) and cos2x = cos(x+x) and see what you get.
  4. Nov 29, 2006 #3


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    So you're telling me that

    ∫_{0}^{π}sin kxsin³xdx =0


    What if k=3 or k=1 ?

  5. Dec 22, 2006 #4
    Actually, you don't even have to go through all that suffering. Whenever you want to find the fourier series of a [tex]\sin^3(x)[/tex] or [tex]\cos^5(x)[/tex] or something, you just have to expand it out as a sum of first order sine and cosine terms--just like you would expand [tex]\sin(x)^[/tex] into [tex]\frac{1-cos(2x)}{2}[/tex].

    In order to expand things like [tex]\sin^3(x)[/tex] or [tex]\cos^5(x)[/tex], just replace the sine and cosine term with its standard complex definition i.e.

    [tex]\cos z=\frac{e^{iz}+e^{-iz}}2[/tex]

    [tex]\sin z=\frac{e^{iz}-e^{-iz}}{2i}[/tex]

    and then just simplify and convert everything back to rectangular form.
  6. Dec 22, 2006 #5


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    Use trig identities to write it as a "trigonometric polynomial". [itex]sin^2(x)= \frac{1}{2}(1- cos(2x))[/itex] so [itex]sin^3(x)= \frac{1}{2}(sin(x)- sin(x)cos(2x))[/itex]. Since sin(a+b)= sin(a)cos(b)+ cos(a)sin(b) and sin(a-b)= sin(a)cos(b)- cos(a)sin(b), sin(a)cos(b)= (1/2)(sin(a+b)+ sin(a-b)) and, in particular, sin(x)cos(2x)= (1/2)(sin(3x)- sin(x)). That is, [itex]sin^3(x)= \frac{1}{2}sin(x)- \frac{1}{4}(sin(3x)- sin(x)= \frac{3}{4}sin(x)- \frac{1}{4}sin(3x)[/itex] (modulo arithmetic errors!).
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