m) Ta có
$x^4 + 2021x^2 - 2020x + 2021 = (x^4 - x^3 + x^2) + (x^3 -x^2 + x) + (2021x^2 -2021x + 2021)$
$= x^2(x^2 - x + 1) + x(x^2 - x + 1) + 2021(x^2 - x + 1)$
$= (x^2 - x + 1)(x^2 + x + 2021)$
n) Ta có
$x^3 + y^3 + z^3 - 3xyz = (x+y)^3 + z^3 - 3xyz - 3xy(x+y)$
$= (x+y+z)[(x+y)^2 + z^2 - z(x+y)] - 3xy(x + y + z)$
$= (x+y+z)(x^2 + y^2 + z^2 + 2xy - xz - zy - 3xy)$
$= (x+y+z)(x^2 + y^2 + z^2 - xy - yz - zx)$
