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Homework Help: Simple Limits Proof

  1. Sep 28, 2010 #1
    1. The problem statement, all variables and given/known data
    Prove:
    "lim f(x) as x-->a is equal to lim f(a+h) as h-->0 (this is really just an exercise in understanding what the terms are)"


    2. Relevant equations



    3. The attempt at a solution

    Now the easiest way would just to pop in the values and boom, lim f(a) = lim f(a). But I think he is looking for a different reasoning. My attempt was to kind of define the statements like so:

    _______

    LHS: f can be made to be as close to a limit L as desired by making x sufficiently close to a.

    RHS: f can be made to be as close to a limit L as desired by making h sufficiently close to zero.

    And in this example, it just so happens when h is made to go to zero we are left with the respective equations L's equal to each other.
    ______

    I don't think I've really "proved" anything though, have I?

    Thanks!
     
  2. jcsd
  3. Sep 28, 2010 #2
    x + a - a = x correct? Let's h = x - a. that means:

    x = h + a
    h = x - a

    also you should know this fact, lim x-->a is the smae as x+b-->a+b
    For example the limit as x tends to 2, is the same as the limit as x+2 tends to 4.
    so x->a is the same as x-a->a-a.

    Sub these into: f(x) as x-->a
    and you get f(h+a) as x-a -> a-a
    which is equivlent to f(h+a) as h-> 0
     
  4. Sep 28, 2010 #3

    fzero

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    You're probably expected to go back to the [tex]\epsilon,\delta[/tex] definition of the limit and use those techniques.
     
  5. Sep 28, 2010 #4
    figured he didn't have to do that since he said
    but a change of variables is rigorous and i'm pretty sure if you went back to δ ε you'd just do the same idea there.
     
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