Define differentiability of a function at a point.

Explanation

A real value function f(x) is defined on `(a,b)` is said to be differentiable at `x=c∈(a,b)` if and only if,
`lim_(x->c)(f(x)-f(c))/(x-c)` exist finitely.
`=>lim_(x->c^-)(f(x)-f(c))/(x-c)=lim_(x->c^+)(f(x)-f(c))/(x-c)`
`=>lim_(h->0)(f(c-h)-f(c))/(-h)=lim_(h->0)(f(c+h)-f(c))/(h)`
=>`LHD=RHD`