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Homework Help: Exponential Integration Question

  1. Jan 27, 2012 #1
    Why is [itex]e^{\int \frac{dt}{t}}[/itex] = [itex]e^{ln|t|}[/itex] = t as apposed to |t|? I don't understand what happened to the absolute value operator. Thanks for any help.

    I understand that [itex]e^{x}[/itex]>0. Is this the justification? But I don't understand why you can't have a negative t in [itex]e^{ln|t|}[/itex] because you would take the absolute value of a negative number.
  2. jcsd
  3. Jan 27, 2012 #2


    Staff: Mentor

    It should be |t|, as you thought.

    It's possible that there is some other context that you're not including, in which t is assumed to be positive. In that case, |t| = t.
  4. Jan 28, 2012 #3
    If I had the differential equation

    [itex]\frac{dy}{dt}[/itex] + [itex]\frac{y}{t}[/itex] = 5

    Then using integration factors

    y = [itex]\frac{5∫e^{\int \frac{dt}{t}}dt}{e^{\int \frac{dt}{t}}}[/itex] = [itex]\frac{5∫e^{ln|t|}dt}{e^{ln|t|}}[/itex] = [itex]\frac{5∫|t|dt}{|t|}[/itex]

    I'm unsure how to proceed without ignoring the absolute value functions but it appears ignoring them seems to be just fine for whatever reason
  5. Jan 29, 2012 #4
    bump - went onto second page
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