Đáp án:
$\begin{array}{l}
a)P = M - N\\
= \dfrac{2}{3}{x^2} - 2xy - 2{x^2}y - y - 1\\
- \left( { - 2{x^2}y + \dfrac{2}{3}{x^2} - 2xy - 2x} \right)\\
= \dfrac{2}{3}{x^2} - 2xy - 2{x^2}y - y - 1\\
+ 2{x^2}y - \dfrac{2}{3}{x^2} + 2xy + 2x\\
= \left( {\dfrac{2}{3}{x^2} - \dfrac{2}{3}{x^2}} \right) + \left( { - 2xy + 2xy} \right)\\
+ \left( { - 2{x^2}y + 2{x^2}y} \right) + 2x - y - 1\\
= 2xy - y - 1\\
b)2x = 3y\\
\Rightarrow x = \dfrac{3}{2}y\,\,\,thay\,vao:x - y = - 1\\
\Rightarrow \dfrac{3}{2}y - y = - 1\\
\Rightarrow \dfrac{1}{2}.y = - 1\\
\Rightarrow y = - 2\\
\Rightarrow x = \dfrac{3}{2}.y = - 3\\
P = 2xy - y - 1\\
= 2.\left( { - 3} \right).\left( { - 2} \right) - \left( { - 2} \right) - 1\\
= 14 + 2 - 1\\
= 15
\end{array}$
