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Rewriting an equation

  1. Apr 27, 2009 #1
    Good day.

    I want to rewrite the following equation:
    [tex]0.5p_{ss}=p_{ss}(1-e^{\frac{Lp}{Ix}t})[/tex]

    What I do is:
    [tex]ln(0.5)=ln(1)-ln(e^{\frac{Lp}{Ix}t})[/tex]
    [tex]ln(0.5)=-{\frac{Lp}{Ix}t}[/tex]

    Though in the notes received it says:
    [tex]ln(0.5)={\frac{Lp}{Ix}t}[/tex] (without the minus sign)

    Am I doing something wrong?
     
  2. jcsd
  3. Apr 27, 2009 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Rewriting

    So you mean you have 0.5pss= pss(1- e((Lp/Ix)t)) and you want to "rewrite the equation"? Rewrite it how? Solve the equation for pss or perhaps Lp or Ix or t?

    You then have ln(0.5)= ln(1)- ln(e((Lp/Ix)t)). How did you get that? Not by taking the logarithm of both sides of the first equation. That would be ln(0.5p)= ln(pss(1- e[sup[(Lp/Ix)t)[/sup]) which reduces to ln(0.5)+ ln(p)=ln(p)+ ln(1- e((Lp/Ix)t)) and the "ln(p)" terms cancel to give ln(0.5)= ln(1- e((Lp/Ix)t)) but ln(1- e((Lp/Ix)t)) is NOT ln(1)- ln(e((Lp)/Ix)t)): in general ln(a- b) is NOT ln(a)- ln(b). If I new HOW you were trying to "rewrite" the equation, I might be able to suggest reducing before you take the logarithm.
     
  4. Apr 27, 2009 #3
    Re: Rewriting

    Thank you for your answer, I want to solve the equation for t.

    0.5pss= pss(1- e((Lp/Ix)t))

    Is the same as:
    0.5= 1- e((Lp/Ix)t)

    Isn't it?

    I don't know how to continue to solve the equation for t...
     
  5. Apr 27, 2009 #4

    Mark44

    Staff: Mentor

    Re: Rewriting

    Add -1 to both sides, multiply both sides by -1, and then take the natural log of both sides.
     
  6. Apr 27, 2009 #5
    Re: Rewriting

    That makes sense, thanks a lot!
     
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