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Homework Help: Calculus Limit Question Help Needed

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

    Limit: The square root of X^2-3X over 2X+1 as X is going towards negative infinity.

    2. Relevant equations

    The square root of X^2-3X over 2X+1 as X is going towards negative infinity.

    3. The attempt at a solution

    I tried to solve it but I have no idea what to do with the negative infinity part. I mean, if it was one number, I would just sub in that number and maybe factor, I am totally lost!
  2. jcsd
  3. Sep 13, 2011 #2


    Staff: Mentor

    [tex]\sqrt{x^2 - 3x} = |x|\sqrt{1 - 3/x}[/tex]
    Does that help?
  4. Sep 13, 2011 #3
    This: √X^2-3X
    lim x→-∞ _________
    Divided by 2X+1

    Like that ^
  5. Sep 13, 2011 #4


    Staff: Mentor

    Yes, I understand what the problem is. I was suggesting something that might help you with it.
  6. Sep 13, 2011 #5
    WHy can't you use L'Hospital's rule?
  7. Sep 13, 2011 #6
    Oh sorry :( lol, I thought you interpreted it wrong.

    I've tried just subing a random number, which was 4 and got 2/9... not sure what to do next.
  8. Sep 13, 2011 #7
    multiply by the conjugate

    it is like multiplying by 1, which is not ilegal, but it will cancel out things to you and make life easier
  9. Sep 13, 2011 #8
    can you show me?
  10. Sep 13, 2011 #9
    Never mind, a friend who took BC cal last year helped me through AIM.

    I got it :) Less work than needed too :)
  11. Sep 13, 2011 #10


    Staff: Mentor

    We are not here to do your work for you. Several people have suggested things to try. Pick one of the techniques and see what you get.

    If you haven't taken a close look at the rules of Physics Forums (which you agreed to abide by when you joined it), you can find them here: https://www.physicsforums.com/showthread.php?t=414380

    The section on Homework Help is applicable here.
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