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Homework Help: Limit at infinity, no vert asymptote, why?

  1. Sep 13, 2011 #1
    1. The problem statement, all variables and given/known data

    Find the vertical asymptote(n) and evaluate the limit as [itex]x \rightarrow n^-, x\rightarrow n^+[/itex], or state Does Not Exist.

    2. Relevant equations

    3. The attempt at a solution
    I have taken the limits at [itex]\pm\infty=2,-2[/itex] and understand those are my horizontal asymptotes.

    I have also finished the problem and got the correct answer (DNE), but I cannot mathematically understand why it does not have a vertical asymptote, I figured this out by graphing.

    Based on similar problems, I (wrongly) assumed setting the denominator equal to 0 gives the Vert Asymptote. This was the case in the 3 problems before this one, but that was after factoring and reducing the equations. After trying to find the limit of this equation as [itex]x\rightarrow1[/itex]...I gave up, and graphed it.

    I don't feel this is the correct approach. What is a better approach? Can I find that this problem has no vertical asymptote without graphing?

    ...After typing this I think my answer lies in the definition of a vertical asymptote, and since the limit of f(x) as x->1 was not [itex]\pm\infty[/itex], then there is no vert. asymptote.

    Is that correct?
  2. jcsd
  3. Sep 13, 2011 #2


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    Science Advisor
    Homework Helper

    Sure, it's not a vertical asymptote because the limit as x->1 is not infinity. To work this out algebraically multiply the numerator and denominator by sqrt(4x^2+2x+10)+4 and factor. It's the usual 'conjugate' thing you should think about when you see a square root.
  4. Sep 13, 2011 #3
    Thanks, I always seem to forget the simple things.
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