AKA 미분계수
the average rate of chage of $\displaystyle y$ with respect to $\displaystyle x$ 가
$\displaystyle \frac{\Delta y}{\Delta x}=\frac{f(x_2)-f(x_1)}{x_2-x_1}$ 일 때,
instantaneous rate of change of $\displaystyle y$ with respect to $\displaystyle x:$$\displaystyle \frac{dy}{dx}=\lim_{\Delta x\to 0}\frac{\Delta y}{\Delta x}$
접선tangent_line과 관련.RENAMETHISPAGE