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Homework Help: Evaluate the limit.

  1. Jan 6, 2008 #1
    1. The problem statement, all variables and given/known data

    Evaluate the Limit.

    lim x -> infinity (3x^3 + x + 26) / (20x^2 - 5x^3)

    2. Relevant equations

    3. The attempt at a solution

    I found the answer to be -3/5. is this correct?
    I just divided the numerator and the denominator by the greatest exponent.
    I see how it was done, but what are the rules to such a problem, and maybe someone could explain what is really going on here. Thankyou.
  2. jcsd
  3. Jan 6, 2008 #2


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    Homework Helper


    to just look and see is the best way. If you wanted, I guess you could have used L'Hopital's Rule:

    f=3x^3 + x + 26
    g=20x^2 - 5x^3

    lim f/g -> inf/inf
    lim f'/g' -> inf/inf
    lim f''/g'' -> inf/inf
    lim f'''/g''' -> -3/5
  4. Jan 6, 2008 #3
    okay i see, but in the original method, do i divide by the largest exponent in the numerator or the denominator, i know I divide the top and bottom by this exponent.
  5. Jan 6, 2008 #4

    Gib Z

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    There is a general method for the limit of the quotient of 2 polynomials. Write out the quotient of a general polynomial of degree m, co efficients are a_m, a_{m-1}..., and then divide by another polynomial degree n, co efficients are b_n, b_{n-1}. There are 3 cases, 1) m > n, m=n and m< n. For every case, divide through by the highest exponent of x. What do you get?
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