`(x-1)(x+4)-(x^2+3x)`

`= x.x+x.4-1.x-1.4-(x^2+3x)`

`= x^2+4x-x-4-x^2-3x`

`= (x^2-x^2)+(4x-x-3x)-4`

`= 0+0-4`

`=-4`

------------------------------

`*** (a+b)(c+d)=ac+ad+bc+bd`

`***` Khi phá ngoặc có dấu trừ đằng trước, ta đổi dấu các hạng tử bên trong `[ (+) to (-); (-) to (+) ]`

`cccolor{#8FBC8F}{~} cccolor{#C1FFC1}{b} cccolor{#B4EEB4}{u} cccolor{#9BCD9B}{i} cccolor{#698B69}{g} cccolor{#2E8B57}{i} cccolor{#54FF9F}{a} cccolor{#4EEE94}{p} cccolor{#43CD80}{h} cccolor{#98FB98}{o} cccolor{#008B45}{n} cccolor{#00FF00}{g} cccolor{#00EE00}{9} cccolor{#00CD00}{9} cccolor{#ADFF2F}{9} cccolor{#228B22}{~}`
            $\color{HotPink}{\heartsuit}$
$\color{pink}{\heartsuit 𝕻𝖍𝖚𝖔𝖓𝖌𝖌 \ 𝕷𝖎𝖓𝖍𝖍 \heartsuit}$