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: Properties of the constant e

  1. Nov 16, 2011 #1
    1. The problem statement, all variables and given/known data
    If the limx→b f(x)=c, then limx→b ex= ec. What property of the function g(x)=ex allows this fact?




    3. The attempt at a solution
    Is it just because e is a constant?
     
    Last edited: Nov 16, 2011
  2. jcsd
  3. Nov 16, 2011 #2
    No. e is a constant, but e^x is not. and probably you meant [itex]lim_{x→b}e^{f(x)}=e^c[/itex]. well, I think the answer is continuity.
     
  4. Nov 16, 2011 #3
    Yes, you're right. I did mean limx→b ef(x)= ec. But how does continuity allow it? Excuse me for being incompetent.
     
  5. Nov 17, 2011 #4

    HallsofIvy

    User Avatar
    Science Advisor

    Did you consider looking up "continuous" in your text?

    That property pretty much is the definition of "continuous":

    The function f(x) is continuous at x= a if and only if
    (1) f(a) exists.
    (2) [itex]\lim_{x\to a} f(x)[/itex] exists.
    (3) [itex]\lim_{x\to a} f(x)= f(a)[/itex].

    Since (3) pretty much implies the left and right sides exist, of only that is given as the definition- but that's really "shorthand".
     
  6. Nov 17, 2011 #5
    well, I meant that the function needs to be continuous at the point x=c. There's a theorem in Calculus that talks about the composition of functions. the theorem states that if [itex]lim_{x→b}f(x)=c[/itex] and [itex]lim_{x→a}g(x)=b[/itex] then [itex]lim_{x→a} fog(x)=c[/itex] is NOT true in general, but this law holds if f is continuous at x=b.
    That's why I said continuity is the key. The theorem seems to be easy to be proved though.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook