Đáp án:
$\begin{array}{l}
b)y\left( {{x^3} + 8} \right) - y\left( {x + 2} \right)\left( {x + 2} \right)\left( {5 - 2x} \right)\\
= y\left( {x + 2} \right)\left( {{x^2} - 2x + 4} \right) - y\left( {x + 2} \right)\left( {5x - 2{x^2} + 10 - 4x} \right)\\
= y\left( {x + 2} \right)\left( {{x^2} - 2x + 4 - 5x + 2{x^2} - 10 + 4x} \right)\\
= y\left( {x + 2} \right).\left( {3{x^2} - 3x - 6} \right)\\
= 3y\left( {x + 2} \right)\left( {{x^2} - x - 2} \right)\\
= 3y\left( {x + 2} \right)\left( {x - 2} \right)\left( {x + 1} \right)\\
c)5{x^4} - 45{x^2}{y^2} - 20{x^3} + 20{x^2}\\
= 5{x^2}\left( {{x^2} - 9{y^2} - 4x + 4} \right)\\
= 5{x^2}\left[ {{{\left( {x - 2} \right)}^2} - 9{y^2}} \right]\\
= 5{x^2}\left( {x - 2 - 3y} \right)\left( {x - 2 + 3y} \right)
\end{array}$
