let $f: J \to \mathbb{R}$ . let $x_n < c <y_n$ be such that $(y_n-x_n) \to0$ show that $\lim _ {n \to \infty} \frac{f(y_n)-f(x_n)}{y_n-x_n}=f'(c)$
Proving that expression is equivalent to the definition of derivative