使用 FOIL 法展開 $$$\left(y - 2\right) \left(y - 5\right)$$$

使用 FOIL 法將 $$$\left(y - 2\right) \left(y - 5\right)$$$ 相乘。

$$$\left({\color{red}\left(y\right)} + {\color{purple}\left(-2\right)}\right) \left({\color{green}\left(y\right)} + {\color{blue}\left(-5\right)}\right) = \underbrace{{\color{red}\left(y\right)} {\color{green}\left(y\right)}}_{\text{第一}} + \underbrace{{\color{red}\left(y\right)} {\color{blue}\left(-5\right)}}_{\text{外部}} + \underbrace{{\color{purple}\left(-2\right)} {\color{green}\left(y\right)}}_{\text{內部}} + \underbrace{{\color{purple}\left(-2\right)} {\color{blue}\left(-5\right)}}_{\text{末頁}} = y^{2} - 5 y - 2 y + 10 = y^{2} - 7 y + 10$$$

$$$\left(y - 2\right) \left(y - 5\right) = y^{2} - 7 y + 10$$$A