Giải thích các bước giải:
$\begin{split}a^2+2ab+b^2-ac-bc&=(a+b)^2-c(a+b)\\&=(a+b)(a+b-c)\end{split}$
$\begin{split}x^2-2x-4y^2-4y&=(x^2-2x+1)-(4y^2+4y+1)\\&=(x-1)^2-(2y-1)^2\\&=(x-2y)(x+2y-2)\end{split}$
$\begin{split}x^2y-x^3-9y+9x&=x^2(y-x)-9(y-x)\\&=(y-x)(x^2-9)\\&=(y-x)(x-3)(x+3)\end{split}$
$\begin{split}x^2(x-1)+16(1-x)&=x^2(x-1)-16(x-1)\\&=(x-1)(x^2-16)\\&=(x-1)(x-4)(x+4)\end{split}$