# Thread: heres a derivative problem for ya!

1. ## heres a derivative problem for ya!

Suppose f is differentiable. Prove that
$f'(x)=\lim_{h\to 0}\frac{f(x+g(h))-f(x)}{g(h)}$ exists, where g is a nonzero function of h such that $\lim_{h\to 0}g(h)=0$ and find the value of $f'(x)$.
I think I need to change the terms of the limit from h to g(h) going to zero, not sure how to though.
Any thoughts?

2. May be that a little problem exist in this case. If $f(*)$ is differentiable in $x=x_{0}$ , then...

$f^{'}(x) = \lim_{h \rightarrow 0+} \frac{f(x_{0}+h)-f(x_{0})}{h}= \lim_{h \rightarrow 0-} \frac{f(x_{0}+h)-f(x_{0})}{h}$ (1)

... i.e. both the derivatives 'from left' and 'from wright' exist and they are equal. In the proposed 'alternative' if , for instance, is $g(h)=h^{2}$ , then is...

$\lim_{h \rightarrow 0} \frac{f(x_{0}+g(h))-f(x_{0})}{g(h)}= \lim_{h \rightarrow 0+} \frac{f(x_{0}+h)-f(x_{0})}{h}$ (2)

... so that we have no information about the derivative 'from left' of $f(*)$ in $x=x_{0}$ ...

Kind regards

$\chi$ $\sigma$