Đáp án:
$\begin{array}{l}
a)\left( {{x^2} + {y^2} + 3x{y^2}} \right) + \left( {{x^2} + {y^2} - 3x{y^2}} \right)\\
= 2{x^2} + 2{y^2}\\
\Leftrightarrow Bậc:2\\
b)\left( {{x^2}y + x{y^2} - \frac{3}{4}x{y^2} + 3y} \right)\\
- \left( {9{x^2} + \frac{2}{5}{y^2} - \frac{1}{2}{x^2}{y^2} - 3x{y^2}} \right)\\
= \frac{1}{2}{x^2}{y^2} + {x^2}y + \left( {1 - \frac{3}{4} + 3} \right)x{y^2}\\
- 9{x^2} - \frac{2}{5}{y^2} + 3y\\
= \frac{1}{2}{x^2}{y^2} + {x^2}y + \frac{{13}}{4}x{y^2} - 9{x^2} - \frac{2}{5}{y^2} + 3y\\
\Leftrightarrow Bậc:4\\
c)2,4x{y^2} + 1,7{y^2} + 2xy - 0,4{x^2} - \left( {1,3{y^2} + 3xy} \right)\\
= 2,4x{y^2} + 0,4{y^2} - 0,4{x^2} - xy\\
\Leftrightarrow Bậc:3\\
d)\left( {{x^3}{y^2} + 3{y^2}} \right) - \left( {{y^2} - \frac{3}{4}{x^3}{y^2}} \right)\\
- \left( {9{x^2}y - 3{y^2}} \right) + \left( {\frac{2}{5}{x^3}{y^2} - \frac{3}{5}{y^2}} \right)\\
= {x^3}{y^2} + 3{y^2} - {y^2} + \frac{3}{4}{x^3}{y^2}\\
- 9{x^2}y + 3{y^2} + \frac{2}{5}{x^3}{y^2} - \frac{3}{5}{y^2}\\
= \frac{{43}}{{20}}{x^3}{y^2} - 9{x^2}y + \frac{{22}}{5}{y^2}\\
\Leftrightarrow Bậc:5
\end{array}$
