We have already seen that the derivative of a function f at x=a is f′(a)=limw→af(w)−f(a)w−a=limh→0f(a+h)−f(a)h.
We can apply the same idea to get a formula
for the derivative of f at every point:f′(◻)=limh→0f(◻+h)−f(◻)h,
where ◻ stands for "anything you like".
In particular, if we take x as input and associate to it f′(x) as output, then we have a function from the set of x-values at which we can compute f′ to the real numbers.
x↦f′(x)