使用 FOIL 法展開 $$$\left(y - 8\right) \left(y + 2\right)$$$

使用 FOIL 法將 $$$\left(y - 8\right) \left(y + 2\right)$$$ 相乘。

$$$\left({\color{red}\left(y\right)} + {\color{purple}\left(-8\right)}\right) \left({\color{green}\left(y\right)} + {\color{blue}\left(2\right)}\right) = \underbrace{{\color{red}\left(y\right)} {\color{green}\left(y\right)}}_{\text{第一}} + \underbrace{{\color{red}\left(y\right)} {\color{blue}\left(2\right)}}_{\text{外部}} + \underbrace{{\color{purple}\left(-8\right)} {\color{green}\left(y\right)}}_{\text{內部}} + \underbrace{{\color{purple}\left(-8\right)} {\color{blue}\left(2\right)}}_{\text{末頁}} = y^{2} + 2 y - 8 y - 16 = y^{2} - 6 y - 16$$$

$$$\left(y - 8\right) \left(y + 2\right) = y^{2} - 6 y - 16$$$A