1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: What is being Done in This proof of Limits?

  1. Jul 10, 2013 #1
    I have been trying to learn calculus by my own, but when it comes to proving limits I get very confuse.

    Could somebody explain me what is being done here?




    If you know any resources that could help me with this task let me know.

    here is the source:
    Last edited: Jul 10, 2013
  2. jcsd
  3. Jul 10, 2013 #2


    User Avatar
    2017 Award

    Staff: Mentor

    The first line is the statement you want to show.
    The second line is a clever guess for delta (as function of epsilon), and the remaining steps are just simplifications, showing that |(5x-4)-6| is indeed < epsilon if |x-2|<delta.
  4. Jul 10, 2013 #3


    Staff: Mentor

    They're starting with this inequality:
    ##|(5x - 4) - 6| < \epsilon##
    In a few algebra operations, they arrive at this:
    ##5|x - 2| < \epsilon ##
    ##|x - 2| < \epsilon/5 ##
    If you let ##\delta = \epsilon/5##, then by reversing the steps above, you'll get back to the first inequality.

    The whole idea is sort of a challenge-response. If you're trying to convince someone that ##\lim_{x \to 2}5x -4 = 6##, they might ask you get a function value within 0.1 (that's the ##\epsilon##). You say, take any x within 0.1/5 = 0.02 of 2.

    If the challenger isn't satisfied, he might ask if you can get the function value within 0.001. You tell him to take any x within 0.0002 of 2 (i.e., between 1.9998 and 2.0002).

    And so on. Eventually, he'll give up and accept that the limit is indeed 2.
    Last edited: Jul 10, 2013
  5. Jul 10, 2013 #4
    Last edited by a moderator: Jul 10, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted