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

Prove lim f(x) as x approaches a is 0?

  1. Sep 21, 2011 #1
    1. The problem statement, all variables and given/known data

    It is given that the limit as x->a of ( f(x)/(x-a) ) is 3. Prove using the espilon-delta theorem of limit that the limit as x->a of f(x) is 0.

    3. The attempt at a solution

    so it is known that: |( f(x)/(x-a) ) - 3| < E1 when |x-a| < d1

    |( f(x)/(x-a) ) - 3| ≤ |f(x)/(x-a)|+3 < E1

    |f(x)| < (E1-3)|x-a|

    (E1-3)|x-a| ≤ C(E-3)

    |x-a| ≤ 1=C

    |f(x)| < (E1-3) when d1=min{ 1, (E-3) }

    and we have to prove that |f(x)-0| < E when |x-a| < d

    so can i just say that:

    |f(x)-0| < E2
    |f(x)| < E2 when |x-a| < d2

    So E2=E1-3 and d2=d1=min{ 1, (E-3) }

    So |f(x)-0| < E1-3 when |x-a| < d1

    is this right? i am not sure if i am making any logical sense in equating the two epsilons?

    my intuition tells me that f(x) is zero at "a" because for f(x)/x-a to have a limit at "a", f(x) must be a factor of (x-a) so as to cancel out with the denominator. so if f(x)=(x-a)(p(x)), then f(a)=0, but i am not sure how to prove that using epsilon-delta, and i am not even sure if f(x) HAS to be a factor of (x-a)...sinx/x after all has a limit at 0 but sinx doesnt cancel with x.

    help? :D
  2. jcsd
  3. Sep 21, 2011 #2


    User Avatar
    Science Advisor

    The denominator of f(x)/(x- 3) goes to 0 as x goes to 3. If f(x) goes to anything other than 0, then the limit of f(x)/(x-3), as x goes to 3, could not be finite.

    It does NOT follow that f(3)= 0 because you are not told that f is continuous there. But that is not asked.
  4. Sep 21, 2011 #3
    why would it not be finite if f(x) did not go to 0?

    specifically i want to know if my proof is right? my professor wants it to be proved using epsilon-delta, not just words..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook