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Homework Help: Infinity Limits

  1. Sep 23, 2007 #1
    ok im confused when x->negative infinity or positive infinity.

    for example

    lim (5x^3+27)/(20x^2 + 10x + 9)
    x-> negative infinty

    heres what i think, i want to know if i have the right idea or not.

    - so since the top exponent is larger then the denominator the lim DNE and so i plugged in a negative value to test if it is negative or positive infinity so i put in -1 just to test, the thing is i only plunged it into the (5x^3)/(20x^2); im thinking this is what i do, and then i get a negative value so i am assuming it is negative infinity. that is what the answer is so posed to be, but did i do it how its so posed to be done?
  2. jcsd
  3. Sep 23, 2007 #2


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    Science Advisor
    Homework Helper

    Divide top and bottom by x^2 and get rid of stuff that tends to zero.
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