PART 2：近似值公式(05：08)

當 \(\Delta x\) 很小，割線斜率 \( \buildrel\textstyle.\over= \) 切線斜率

\(\frac{{f(x + \Delta x) - f(x)}}{{\Delta x}} \buildrel\textstyle.\over= f'(x)\)

\(f(x + \Delta x) \buildrel\textstyle.\over= f(x) + f'(x)\Delta x\)

\(\Delta x\) 愈小，微分估計值愈準確