Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Limits Question Definition

  1. Mar 19, 2008 #1
    If I have a function lets say lim x->5 f(x) that

    f(5) Does not exist, because if I give a value of x which equals the limit , there is no anymore limit?

    I hope the question is understandble.

    Thanks in advance.
  2. jcsd
  3. Mar 19, 2008 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The existence of a limit of a function [itex]f(x)[/itex] at a point [itex]x=c[/itex] has nothing to do with whether the function is actually defined there. That's because a control is built into the definition of a limit that prevents [itex]x[/itex] from taking on the value [itex]c[/itex] as we take the limit. That's what the [itex]0<|x-c|<\delta[/itex] is there for, to ensure that the distance between [itex]x[/itex] and [itex]c[/itex] is strictly positive.
  4. Mar 19, 2008 #3
    Yeah, I like to think of it as making sure that it is approaching c from both sides.

    A good example might be [tex]\frac{\sqrt{x+1} -1}{x}[/tex]. The limit exists at c > 0 and the answer is 0 but the you can't just plug that in.

    if you let x = -0.1 f(x) is 5.132 and -0.01 f(x) .5013, and -0.001 f(x) is .5001 (from the left), and x is .001, f(x) is 0.4999, x is 0.1, f(x) is 0.4988 (from the right), and so on.

    When you study continuity I think it really helps clear things up in regards to the existence of limits.
  5. Mar 19, 2008 #4
    Thank you very much
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook