And now I can write a different formula for the derivative, which is the following: so this f'(x0) , which is the same as m, is going to be the limit as delta x goes to 0 of the change in f, well the change in f is the value of f at the upper point here, which is (x0 delta x), and minus its value at the lower point P, which is f(x0), divided by delta x. 