+ Cho $x$ một số gia $\Delta x = x-x_0 \Rightarrow x= \Delta x +x_0$
+ $\Delta y = f(x) -f(x_0) = f(\Delta x + x_0) -f(x_0) = (x_0 +\Delta x)^2 - 3(x_0 + \Delta x) -x_0^2 +3x_0$
$= \Delta^2 x +2x_0 \Delta x -3\Delta x$
+ $\dfrac{\Delta x}{\Delta y} = \dfrac{\Delta^2 x +2x_0 \Delta x -3\Delta x}{\Delta x}=\Delta x +2x_0 -3$
+ $\lim \limits_{\Delta x \to 0} (\Delta x +2x_0 -3)=2x_0 -3$