# Thread: how to show the following equality holds.

1. ## any hints to solve the following !!

Show that $f(x)$ is the derivative of $f(.)$ at $x$ if and only if $lim_{h \to 0} \sup_{|t|\leqslant h} \frac{|f(x+t)-f(x)-tf^{'} (x)|}{h} = 0$