使用 FOIL 法展開 $$$\left(x + 1\right) \left(x + 1\right)$$$

使用 FOIL 法將 $$$\left(x + 1\right) \left(x + 1\right)$$$ 相乘。

$$$\left({\color{red}\left(x\right)} + {\color{purple}\left(1\right)}\right) \left({\color{green}\left(x\right)} + {\color{blue}\left(1\right)}\right) = \underbrace{{\color{red}\left(x\right)} {\color{green}\left(x\right)}}_{\text{第一}} + \underbrace{{\color{red}\left(x\right)} {\color{blue}\left(1\right)}}_{\text{外部}} + \underbrace{{\color{purple}\left(1\right)} {\color{green}\left(x\right)}}_{\text{內部}} + \underbrace{{\color{purple}\left(1\right)} {\color{blue}\left(1\right)}}_{\text{末頁}} = x^{2} + x + x + 1 = x^{2} + 2 x + 1$$$

$$$\left(x + 1\right) \left(x + 1\right) = x^{2} + 2 x + 1$$$A