Differentiable function

A function f(x) is differentiable at the point a if and only if, as x approaches a (which it is never allowed to reach), the value of the quotient:

${\frac {f(x)-f(a)}{(x-a)}}$

approaches a limiting value that we call the derivative of the function f(x) at x=a.

There is also the more rigorous $\epsilon -\delta$ definition: a function f is said to be differntiable at point a if ∀$\epsilon >0$ ∃​Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta>0} such that if

$|x-a|<\delta \,$

then

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |\frac{f(x) - f(a)}{x-a} - f'(a) | < \epsilon \,} .