Đáp án:
$\begin{array}{l}
a)5{x^2} - 3x\left( {x + 2} \right)\\
= 5{x^2} - 3{x^2} - 6x\\
= 2{x^2} - 6x\\
b)3x\left( {x - 5} \right) - 5x\left( {x + 7} \right)\\
= 3{x^2} - 15x - 5{x^2} - 35x\\
= - 2{x^2} - 50x\\
c)3{x^2}y\left( {2{x^2} - y} \right) - 2{x^2}\left( {2{x^2}y - {y^2}} \right)\\
= 6{x^4}y - 3{x^2}{y^2} - 4{x^4}y + 2{x^2}{y^2}\\
= 2{x^4}y - {x^2}{y^2}\\
d)3{x^2}\left( {2y - 1} \right) - \left[ {2{x^2}\left( {5y - 3} \right) - 2x\left( {x - 1} \right)} \right]\\
= 6{x^2}y - 3{x^2} - \left( {10{x^2}y - 6{x^2} - 2{x^2} + 2x} \right)\\
= 6{x^2}y - 3{x^2} - \left( {10{x^2}y - 8{x^2} + 2x} \right)\\
= - 4{x^2}y + 5{x^2} - 2x\\
e)4x\left( {{x^3} - 4{x^2}} \right) + 2x\left( {2{x^3} - {x^2} + 7x} \right)\\
= 4{x^4} - 16{x^3} + 4{x^4} - 2{x^3} + 14{x^2}\\
= 8{x^4} - 18{x^3} + 14{x^2}\\
f)25x - 4\left( {3x - 1} \right) + 7x\left( {5 - 2{x^2}} \right)\\
= 25x - 12x + 4 + 35x - 14{x^3}\\
= - 14{x^3} + 48x + 4
\end{array}$
