Original formula l (z) = [(x+z) 2-4 (x+z)-x 2+4x]/z.
=(x^2+2zx+z^2-4x-4z-x^2+4x)/z
=(2zx-4z)/z
=2x-4
When z-> 0 o'clock
Original limit =2x-4
In fact, for the original constant formula L (z): [f (x+z)-f (x)]/z when z->; 0 Its limit is the first derivative of f(x) at x point.