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Homework Help: Fourier series problem solving for an,bn

  1. Jul 9, 2011 #1
    1. The problem statement, all variables and given/known data
    let f(x)={0;-2[itex]\leq[/itex]x[itex]\leq[/itex]0.
    x;0[itex]\leq[/itex]x[itex]\leq[/itex]2
    find a0
    an
    bn
    given the period is 4
    2. Relevant equations
    a0=1/L[itex]\int[/itex]f(x)dx
    an=1/L[itex]\int[/itex]f(x)cos(n[itex]\pi[/itex]x/L)
    bn=1/L[itex]\int[/itex]f(x)sin(n[itex]\pi[/itex]x/L)

    3. The attempt at a solution
    so I can get a0 = 1 but I run into trouble with an. so I plug
    an=1/2[itex]\int[/itex]xcos(n[itex]\pi[/itex]x/L) for the interval 0[itex]\leq[/itex]x[itex]\leq[/itex]2 and I get the solution
    =(1/n2[itex]\pi[/itex]2)2xn[itex]\pi[/itex]sin(n[itex]\pi[/itex]x/2)+4cos(n[itex]\pi[/itex]x/2) then subbing in for x I get (1/n2[itex]\pi[/itex]2)(4n[itex]\pi[/itex]sin(n[itex]\pi[/itex])+4cos(n[itex]\pi[/itex])-1) can anyone tell me what I am doing wrong here?
     
  2. jcsd
  3. Jul 9, 2011 #2
    Hi dp182!

    You made two mistakes: you forgot the term 1/L, so you still need to divide by 2. Secondly, when you "subbed for x", you made a mistake in "subbing for 0". That is, the correct formula is

    [tex]\int_0^2{f(x)dx}=F(2)-F(0)[/tex]

    But somehow, you calculated F(0) wrong.
     
  4. Jul 9, 2011 #3
    but wouldn't F(0) be 4cos(0)/n2pi2 which is just 4/n2pi2
     
  5. Jul 9, 2011 #4
    Indeed, but you wrote -1 instead of -4 in the end.
     
  6. Jul 10, 2011 #5
    ya but I brought out a 4/n2pi2 so its not -1 its -4/n2pi2
     
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