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Integrating Factor

  1. Sep 4, 2013 #1
    1. The problem statement, all variables and given/known data
    y' + (2/t)y = (cost)/(t^2), and the following condition is given: y(pi) = 0


    2. Relevant equations



    3. The attempt at a solution
    After employing the integrating factor, I find the solution to be:

    [itex]y=e^{-2t} \int e^{2t} \frac{\cos(t)}{t^2} dt[/itex].

    Evidently, this simplifies all the way to y = (sin t)/(t^2). I am not sure as to how this integral should be solved. Any hints would be much welcomed.
     
  2. jcsd
  3. Sep 4, 2013 #2

    Curious3141

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    I think you should show all your steps, starting with exactly how you applied your integrating factor. Clearly, there's an error somewhere.
     
  4. Sep 4, 2013 #3
    I see what I did wrong.

    mu of t, the integrating factor, [itex]\mu (t) = e^{\int \frac{2}{t}}dt = e^{2 \ln t} = e^{2t}[/itex]

    Do you see where I went wrong? It should be [itex]\mu (t) = 2t[/itex]
     
  5. Sep 4, 2013 #4

    HallsofIvy

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    This is an example of what happens when you memorize formulas (imperfectly) rather than learning basic definitions and how the formulas are derived.

    An "integrating factor" is a function, [itex]\mu(t)[/itex] such that multiplying the equation by it converts the left side into a single derivative. Here, that means we must have [itex]\mu y'+ (2\mu/t)y= (\mu y)'[/itex]. Expanding the derivative on the right that becomes [itex]\mu y'+ \mu' y= \mu y'+ (2\mu/t)y[/itex] which reduces to [itex]\mu'= 2\mu/t[/itex], a separable differential equation for [itex]\mu[/itex]. [itex]d\mu/\mu= 2dt/t[/itex] integrates to [itex]ln(\mu)= 2ln(t)[/itex] or [itex]\mu= t^2[/itex] NOT "2t" (I have neglected the "constant of integration since we only need a single function).

    Multiplying the entire equation by [itex]t^2[/itex] gives [itex]t^2y'+ 2ty= (t^2y)'= cos(t)[/itex] which is easy to integrate.
     
  6. Sep 4, 2013 #5

    Curious3141

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    ##e^{2 \ln t} = (e^{\ln t})^2 = t^2##, that's your integrating factor.

    Your integrating factor is NOT ##2t##. ##2t## is in fact the derivative of your integrating factor, and you should be able to see this from applying product rule to ##yt^2##.
     
    Last edited: Sep 4, 2013
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