使用 FOIL 法展開 $$$\left(b + 3\right) \left(b - 9\right)$$$

使用 FOIL 法將 $$$\left(b + 3\right) \left(b - 9\right)$$$ 相乘。

$$$\left({\color{red}\left(b\right)} + {\color{purple}\left(3\right)}\right) \left({\color{green}\left(b\right)} + {\color{blue}\left(-9\right)}\right) = \underbrace{{\color{red}\left(b\right)} {\color{green}\left(b\right)}}_{\text{第一}} + \underbrace{{\color{red}\left(b\right)} {\color{blue}\left(-9\right)}}_{\text{外部}} + \underbrace{{\color{purple}\left(3\right)} {\color{green}\left(b\right)}}_{\text{內部}} + \underbrace{{\color{purple}\left(3\right)} {\color{blue}\left(-9\right)}}_{\text{末頁}} = b^{2} - 9 b + 3 b - 27 = b^{2} - 6 b - 27$$$

$$$\left(b + 3\right) \left(b - 9\right) = b^{2} - 6 b - 27$$$A