2.
b) $x^3-x+3x^2y+y^3-y\color{red}{+3xy^2}$
$=(x^3+3x^2y+3xy^2+y^3)-(x+y)$
$=(x+y)^3-(x+y)$
$=(x+y)[(x+y)^2-1]$
$=(x+y)(x^2+y^2+2xy-1)$
c) $5x^2-10xy+5y^2-20z^2$
$=5(x^2-2xy+y^2-4z^2)$
$=5[(x-y)^2-4z^2]$
$=5(x-y-2z)(x-y+2z)$
đ) $2x^2+3x-5$
$=2x^2-2x+5x-5$
$=2x(x-1)+5(x-1)$
$=(x-1)(2x+5)$
4. a)...