Đáp án:
$\begin{array}{l}
a) - 4{x^5}\left( {{x^3} - 4{x^2} + 7x - 3} \right)\\
= - 4{x^8} + 16{x^7} - 28{x^6} + 12{x^5}\\
b) - 4x\left( {3x - 4} \right) + 7x\left( {x - 5} \right)\\
= - 12{x^2} + 16x + 7{x^2} - 35x\\
= - 5{x^2} - 19x\\
c)\left( {2x + 3y} \right)\left( {3x + 2y - 1} \right)\\
= 6{x^2} + 4xy - 2x + 9xy + 6{y^2} - 3y\\
= 6{x^2} + 6{y^2} - 2x - 3y + 13xy\\
d)A = \left( {5x - 7} \right)\left( {2x + 3} \right) - \left( {7x + 2} \right)\left( {x - 4} \right)\\
= 10{x^2} + 15x - 14x - 21 - \left( {7{x^2} - 28x + 2x - 8} \right)\\
= 10{x^2} + x - 21 - 7{x^2} + 26x + 8\\
= 3{x^2} + 27x - 13\\
= 3.{\left( {\dfrac{1}{2}} \right)^2} + 27.\dfrac{1}{2} - 13\\
= 3.\dfrac{1}{4} + \dfrac{{27}}{2} - 13\\
= \dfrac{5}{4}
\end{array}$
