Đáp án:
$\begin{array}{l}
a)\left( { - 3{x^n}} \right)\left( {{x^{n - 1}} + \frac{1}{3}x} \right).n\\
= - 3{x^{n + n - 1}}.n - {x^{n + 1}}.n\\
= - 3n.{x^{2n - 1}} - n.{x^{n + 1}}\\
b)3{x^n}.{y^n}.\left( {13{x^2}y - 5x{y^2} + 6{x^2}{y^2}} \right)\\
= 39{x^{n + 2}}.{y^{n + 1}} - 15{x^{n + 1}}.{y^{n + 2}} + 18{x^{n + 2}}.{y^{n + 2}}\\
c)\left( {x - y} \right)\left( {{x^2} + xy + {y^2}} \right)\\
= {x^3} - {y^3}\\
d)\left( {5{x^2}y - 2x{y^2} + {y^2}} \right).\left( { - {x^3} - 2{x^2}y + 5x{y^2}} \right)\\
= - 5{x^5}y - 10{x^4}{y^2} + 25{x^3}{y^3}\\
+ 2{x^4}{y^2} + 4{x^3}{y^3} - 10{x^2}{y^4}\\
- {x^3}{y^2} - 2{x^2}{y^3} + 5x{y^5}\\
= - 5{x^5}y - 8{x^4}{y^2} + 29{x^3}{y^3} - 10{x^2}{y^4}\\
- {x^3}{y^2} - 2{x^2}{y^3} + 5x{y^5}
\end{array}$
