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Lim(x->inf) of ((x+a)/(x-a))^x = e

  1. Aug 3, 2005 #1
    lim(x-->inf) of ((x+a)/(x-a))^x = e

    I started this problem and quickly became stuck, the question asks for what value of "a" is the following true:

    lim(x-->inf) of ((x+a)/(x-a))^x = e

    I took the natural log of both sides to start and got this:

    lim(x-->inf) of x*ln((x+a)/(x-a)) = 1

    I've tried going on from here but nothing in the end makes sense and i don't know any other way to start the problem; any help is appriciated, thanks.
    Last edited: Aug 3, 2005
  2. jcsd
  3. Aug 3, 2005 #2
    The text is wrong:

    [tex]\lim_{\substack{x\rightarrow 0}}f(x) = 1 , \forall a \in \mathbb{R}[/tex]


    [tex]\lim_{\substack{x\rightarrow \infty}}f(x) = e^{2a}[/tex]

    Last edited: Aug 3, 2005
  4. Aug 3, 2005 #3
    ah, yes, i did mean for x to go to infinity; but how did you get e^2a?
  5. Aug 3, 2005 #4
    [tex]\lim_{\substack{ x \rightarrow \infty}} {(\frac {x+a}{x-a})}^x = \lim_{\substack{ x \rightarrow \infty}} {(1+ \frac {2a}{x-a})}^x = \\
    \lim_{\substack{ y \rightarrow \infty}} {(1+ \frac {2a}{y})}^{y+a}= [/tex]
    [tex]\lim_{\substack{y \rightarrow \infty}} {(1+ \frac {2a}{y})}^y {(1+ \frac {2a}{y})}^a =
    \\ \lim_{\substack{y\rightarrow \infty}}{(1+ \frac {2a}{y})}^y = e^{2a}[/tex]
    Last edited: Aug 3, 2005
  6. Aug 3, 2005 #5
    thank you, i think i have it now
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