Đáp án:
$\begin{array}{l}
a)\left( { - 2 - 7b} \right).\left( {15 - {b^2} + {b^5}} \right)\\
= - 30 + 2{b^2} - 2{b^5} - 105b + 7{b^3} - 7{b^6}\\
b)\left( {{d^2} + 3cd + 9{c^2}} \right)\left( {d - 3c} \right)\\
= {d^3} - {\left( {3c} \right)^3}\\
= {d^3} - 27{c^3}\\
c)x\left( {x + 2} \right).\left( { - 2x + 3} \right)\\
= \left( {{x^2} + 2x} \right)\left( { - 2x + 3} \right)\\
= - 2{x^3} + 3{x^2} - 4{x^2} + 6x\\
= - 2{x^3} - {x^2} + 6x\\
d)\dfrac{1}{2}{x^2}\left( { - x + 7} \right)\left( { - x - 7} \right)\\
= \dfrac{1}{2}{x^2}\left( {x - 7} \right)\left( {x + 7} \right)\\
= \dfrac{1}{2}{x^2}\left( {{x^2} - 49} \right)\\
= \dfrac{1}{2}{x^4} - \dfrac{{49}}{2}{x^2}
\end{array}$
