DifferentiableThe function is said to be differentiable at every x for which this limit exists, and
its derivative may be denoted by f ′(x), y′, , or Dxy. Frequently ∆x is replaced
by h or some other symbol.
The derivative of y = f (x) at x = a, denoted by f ′(a) or y′(a), may be defined as
follows: