/sci/ - Science & Math

Why?
I've got
[eqn] \lim\limits_{x\to x_0}\dfrac{f(x)-f(x_0)}{x-x_0}=f'(x_0)\,; \\
\lim\limits_{x\to x_0}\dfrac{f(x)-f(x_0)-f'(x_0)(x-x_0)}{x-x_0}=0 [/eqn]
but i can't see where to go from there, and i feel like i'm missing the o() part.