Đáp án:
$\begin{array}{l}
a)5{x^2}y - 20x{y^2}\\
= 5xy\left( {x - 4y} \right)\\
b){a^2}\left( {a - x} \right) - ay + xy\\
= {a^2}\left( {a - x} \right) - y\left( {a - x} \right)\\
= \left( {a - x} \right)\left( {{a^2} - y} \right)\\
c)5{x^2} + 10x + 5 - 5{y^2}\\
= 5\left( {{x^2} + 2x + 1 - {y^2}} \right)\\
= 5\left[ {{{\left( {x + 1} \right)}^2} - {y^2}} \right]\\
= 5.\left( {x + 1 + y} \right)\left( {x + 1 - y} \right)\\
d)2xy - {x^2} - {y^2} + 16\\
= 16 - \left( {{x^2} - 2xy + {y^2}} \right)\\
= {4^2} - {\left( {x - y} \right)^2}\\
= \left( {4 - x + y} \right)\left( {4 + x - y} \right)\\
e)3{x^3} - {x^2} - 4x\\
= x\left( {3{x^2} - x - 4} \right)\\
= x\left( {3{x^2} + 3x - 4x - 4} \right)\\
= x\left( {3x - 4} \right)\left( {x + 1} \right)
\end{array}$
