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

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

$$$\left({\color{red}\left(x\right)} + {\color{purple}\left(5\right)}\right) \left({\color{green}\left(x\right)} + {\color{blue}\left(-2\right)}\right) = \underbrace{{\color{red}\left(x\right)} {\color{green}\left(x\right)}}_{\text{第一}} + \underbrace{{\color{red}\left(x\right)} {\color{blue}\left(-2\right)}}_{\text{外部}} + \underbrace{{\color{purple}\left(5\right)} {\color{green}\left(x\right)}}_{\text{內部}} + \underbrace{{\color{purple}\left(5\right)} {\color{blue}\left(-2\right)}}_{\text{末頁}} = x^{2} - 2 x + 5 x - 10 = x^{2} + 3 x - 10$$$

$$$\left(x + 5\right) \left(x - 2\right) = x^{2} + 3 x - 10$$$A