a)
$P(x)=(x^3-3x^2+2x+1).(-x^2)-x(2x^2-3x+1)$
$P(x)=-x^5+3x^4-2x^3-x^2-2x^3+3x^2-x$
$P(x)=-x^5+3x^4+(-2x^3-2x^3)+(-x^2+3x^2)-x$
$P(x)=-x^5+3x^4-4x^3+2x^2-x$
Hệ số $x^3$ là $-4$
b)
$Q(x) = [5x^2 - a(x+a)] - [3(a^2 - x^2)+2ax] + [2ax-4(a+2ax^2)$
$Q(x)=5x^2-ax-a^2-3a^2+3x^2-2ax+2ax-4a-8ax^2$
$Q=8x^2-ax-4a^2-4a-8ax^2$
Hệ số $x^3$ là $0$
xin hay nhất ạ !