Ok, I know how you feel about homework threads… I’m not looking for
answers, I’m looking for a direction here.
My calc I teacher gave us this project to do. We’re given some
information, and we have to do some things with it, in 3 separate
mini-projects. Here’s what I have, and my ideas on it.
Information
1: f(a+b)=f(a)f(b)
2: f(0)=0
3: f(x) is differentiable at x=0, and f(0)=1
Regardless of my response to project 1, the only things I know about a
function f(x) are properties 1-3, IOW, my answer for project 1 doesn’t
matter when doing the other two projects.
Projects
1: Find a function f(x) that satisfies all of the above conditions
So I think I’ve succeded here, with a simple f(x)=e[sup]x[/sup]
function. This isn’t where I’m having the problems.
2: Show that condition 1 implies that either f(0)=0 or f(0)=1. Then
show that if f(0)=0 then f(x)=0 for all x, while if f(0)=1 then f(x)
!=0 for all x.
Huh? Here, the only thing I’ve been able to think of that might work
would be the Mean Value Theorem, using f(0)=0 for my first point, but
I don’t have a 2nd point to define the interval on. Is this even the
right direction to go here? Would saying something like f(0+0)=2f(0)
help there?
- Using the limit definition of a derivative, show that
f’(x)=f(x)f’(0) for all x. What does this say about the continuity of
f(x) for all x?
I think that here I need to use the product rule, by saying
f(x+0)=f(x)f(0), and then using the product rule to say that
f’(x+0)=f’(x)f(0)+f(x)f’(0), and since information point 2 says that
f(0)=0, then the first term goes away. I think I’m on the right track
there, but I don’t know what to do to say anything about the
continuity. Is it as simple as saying that since f(x) is
differentiable over all x, then it’s continuous over all x?