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Homework Help: Limits: Power Function, can't get same answer!

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

    The question asks to find the limit as x approaches 8 of (2x^2 - 3x + 4).
    So I use the limit law/property which states that if f is a polynomial or rational function and if a is in the domain of f, then the limit of f(x) as x approaches a is f(a). Correct?

    2. Relevant equations:

    Now, since the limit law I described clearly fits as this is a polynomial expression, and x can be any real number, I plug in 8, aka f(a) = f(8). The answer I get does not agree with the answer key.

    3. The attempt at a solution:

    Calculus never came easy for me, but here is my attempt:
    lim x->8 (2x^2 - 3x + 4)
    = 2(8)^2 - 3(8) + 4
    = 108
    Do I even have the right approach??
    Here's the answer key:
    lim x->8 (2x^2 - 3x + 4)
    = 2(5)^2 - 3(5) + 4
    = 39

    Assuming no error in the actual substitutions & calculation, where the heck did the key get 5??? Am I missing something here, or is the answer key throwing out a red herring? 99.99% of the time, whenever the answer key disagrees with my answer: I did something wrong.
    What happened this time? It seems like that instead of using 8, they used 5... why?
  2. jcsd
  3. Oct 6, 2008 #2


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    Homework Helper

    your work with [tex] 8 [/tex] is correct.

    One of two possiilities:
    1) The question was written with 8 when it should have been 5
    2) The answer was written with 5 when it should have been 8

    A minor chance - you are looking at the wrong solution, but since the polynomial matches the chance of that seems very low.
  4. Oct 7, 2008 #3
    Okay, thanks statdad.

    I figured it was something like that, but because this is all so new to me, I couldn't be sure.

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