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Limit of natural log

  1. Feb 12, 2012 #1
    Determine the infinite limit.
    lim x->4+ ln(x^2-16)

    I know from graphing the equation and doing a table that the limit is -infinity, but my book is saying to do the following.

    Let t = x^2-16, Then as x->4+, t->0+, and lim x->4+ ln(x^2-16)=lim t->0+ ln(t) by 3
     
  2. jcsd
  3. Feb 12, 2012 #2
    I think it is just substituting in t for the function inside the natural log to make it easier to understand. Putting 4 into the equation you get 42-16=16-16=0 so it is the limit of ln(0). Thinking about ln(0), it is undefined as it represents the number you would need to take e to the power of to get ex=0. Since this ex can never reach 0 but only approach it, ln(0) is undefined and you limit is, as you thought, -∞.

    Unless I'm totally wrong.
     
  4. Feb 12, 2012 #3
    ah i see. i guess that makes sense. It just seems rather pointless lol.
     
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