Đáp án:
$\begin{array}{l}
a)x\left( {{x^2} - y} \right) - {x^2}\left( {x + y} \right) + y\left( {{x^2} - y} \right)\\
= \left( {{x^2} - y} \right)\left( {x + y} \right) - {x^2}\left( {x + y} \right)\\
= \left( {{x^2} - y - {x^2}} \right)\left( {x + y} \right)\\
= - y.\left( {x + y} \right)\\
= - 100.\left( {\dfrac{1}{2} - 100} \right)\\
= - 100.\dfrac{{ - 199}}{2}\\
= 50.199\\
= 9950\\
b)\\
5x\left( {x - 4y} \right) - 4y\left( {y - 5} \right)\\
= 5{x^2} - 20xy - 4{y^2} + 20y\\
= 5.{\left( {\dfrac{{ - 1}}{5}} \right)^2} - 20.\dfrac{{ - 1}}{5}.\dfrac{{ - 1}}{2} - 4.{\left( {\dfrac{{ - 1}}{2}} \right)^2} + 20.\dfrac{{ - 1}}{2}\\
= \dfrac{1}{5} - 2 - 1 - 10\\
= \dfrac{1}{5} - 13\\
= - \dfrac{{64}}{5}
\end{array}$
