We have already seen that the derivative of a function f at
x=a is f′(a)=limx→af(x)−f(a)x−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". Recall that we said you will need to be able to
compute this second form of the derivative, with the limit as
h→0. That is the only form we can use to compute the
derivative function.
In particular, if we take x as input and associate to it
f′(x) as output, then we have a function.
We see how to compute f′(x) in the video. Fortunately, you
already know how to do this, as you will see.