Given f(x) is continuos at x0, for f(x) to be differentiable at x0, the left hard Derivative and the right hand Derivative must exist fanitely.
For f(x) to be differentiable at x0, the
LHD, lim△→0f(x+△x)−f(x)△x and
RHD, lim△→0f(x+△x)−f(x)△x must be finite and equal.
For example, the graph f(x) has LHD and RHS but are not equal.
Intestively whenever there is a sharp edge on f(x) the LHD and RHD are not equal and f(x) wont be differentiable at x0 even though given f(x) is continuous