Đáp án:
$\begin{array}{l}
a)A = 5x\left( {4{x^2} - 2x + 1} \right) - 2x\left( {10{x^2} - 5x - 2} \right)\\
= 20{x^3} - 10{x^2} + 5x - 20{x^3} + 10{x^2} + 4x\\
= 9x\\
= 9.15\\
= 135\\
b)\\
B = 5x\left( {x - 4y} \right) - 4y\left( {y - 5x} \right)\\
= 5{x^2} - 20xy - 4{y^2} + 20xy\\
= 5{x^2} - 4{y^2}\\
= 5.{\left( {\dfrac{{ - 1}}{5}} \right)^2} - 4.{\left( {\dfrac{{ - 1}}{2}} \right)^2}\\
= 5.\dfrac{1}{{25}} - 4.\dfrac{1}{4}\\
= \dfrac{1}{5} - 1\\
= - \dfrac{4}{5}
\end{array}$
