1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding limit of f(x) =

  1. Dec 12, 2012 #1

    939

    User Avatar

    1. The problem statement, all variables and given/known data

    I cannot remember how to do this. How do you find the limit of expressions, without graphs of them?

    2. Relevant equations

    i.e.

    lim x -> 3 (x^2 - 5x + 2)

    3. The attempt at a solution

    1) (3 + h)^2 - 5(3 + h) + 2
    = (3 + h)(3 + h) - 15 - 5h + 2
    9 + 6h - h^2 - 15 - 5h + 2
    -4 - 1h - h^2
    ...
    The book says it is -4, but is this the right method? Will it work for rational expressions too?
     
  2. jcsd
  3. Dec 12, 2012 #2

    Mark44

    Staff: Mentor

    f(x) = x2 - 5x + 2 is a polynomial, hence it is continuous everywhere. To find the limit of f as x → 3, simply evaluate f(3). You should get -4.

    It looks like you are misremembering part of the difference formula in the definition of the derivative.
     
  4. Dec 12, 2012 #3

    939

    User Avatar

    Thanks, got it.

    In the case of a rational expression, I merely:

    1) Evaluate at f(#)
    2) If that = 0, try to see if it can be factored and then evaluate again

    Correct?
     
  5. Dec 12, 2012 #4

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    Yes. But if f(x)=(x-1)/(x^2-1) and you want the limit as x->1, you don't get 0. You get 0/0. That's the danger sign. But you've got the idea.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Finding limit of f(x) =
  1. Limit f(x,y) (Replies: 8)

  2. Limit of f(x,y) (Replies: 10)

Loading...