Question

Someone explains me, please. Question on comment

Answer 1

@wio @tkhunny

Answer 2

It looks to me like in the line you are having problems with, your teacher has just stated the definition of the derivative by first principle, however, applied it to your problem. That is:

\[\lim_{h \rightarrow 0}\frac{f(a+h)-f(a)}{h}\]

Answer 3

ah, ok, I got it. thanks a lot