TITLE: Taylor's Theorem Application Question, $f(x)$ smooth and $f(0)=0$ implies $f(x)/x$ smooth.
QUESTION [5 upvotes]: I am wondering the following fact, and I believe I know the answer, but I am not sure why.
If $f(x)$ is a smooth function from $\mathbb{R}$ to $\mathbb{R}$, if $f(0)=0$, is it true that $f(x)/x$ is smooth?
I believe this is an application of Taylor's theorem (a text I was reading used Taylor's theorem at some part of a proof, and I believe it to be this part), however, I do not see why.
Any help is much appreciated.

REPLY [3 votes]: Taylor's theorem with integral remainder at order 0 (this is a very pendantic way of calling the fundamental theorem of calculus) yields
$$
f(x) = f(0) + \int_0^x f'(t)\,dt = f(0) + x\int_0^1f'(ux)\,du
$$
Hence using $f(0)=0$,
$$
\frac{f(x)}{x} = \int_0^1 f'(ux)du
$$
is smooth.