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Homework Help: Limits again

  1. May 20, 2008 #1
    Problem: Find the limit of ln(1+e^x)/e^x as x approaches negative infinity

    I really have no idea how to do this kind of limit, so I guessed. I tried to substitute u=e^x to get ln(1+u)/u as u approaches negative infinity, then applying l'hopital's rule and eventually ended up with 1/negative infinity which is zero. This wasn't the right answer (which was 1). Appearently what I did wasn't the right thing, so what is the right way for this kind of problem?
     
  2. jcsd
  3. May 20, 2008 #2
    As x goes to infinity, we have an indeterminate form of 0/0. So apply L'Hopital's rule.

    Show us your work because using L'Hopital should have worked because you messed up your algebra.
     
  4. May 20, 2008 #3
    Ok, substituting u = e^x

    lim ln(1+u)/u as u approaches negative infinity.

    Applying lhopital's rule we have

    1/(u+1)/1

    so the limit of 1/(u+1) as u approaches negative infinity = 1/(negative infinity+1) = 1/negative infinity = 0

    Which part did I mess up?
     
  5. May 20, 2008 #4
    As x goes to negative infinity, what does u go to?
     
  6. May 20, 2008 #5
    Quit substituting, you don't need to at all.

    [tex]\lim_{x\rightarrow-\infty}\frac{\ln{(1+e^x)}}{e^x}[/tex]

    [tex]\lim_{x\rightarrow-\infty}\frac{\frac{e^x}{1+e^x}}{e^x}[/tex]

    [tex]\lim_{x\rightarrow-\infty}\frac{1}{1+e^x}[/tex]

    Answer ...
     
    Last edited: May 20, 2008
  7. May 20, 2008 #6
    Right, thanks.
     
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