Đáp án:
$\begin{array}{l}
a)A = 5x{y^2} + xy - x{y^2} - \frac{1}{3}{x^2}y + 2xy\\
+ {x^2}y + xy + 6\\
A = 4x{y^2} + \frac{2}{3}{x^2}y + 4xy + 6\\
\Rightarrow Bậc:3\\
b)A + B = 0\\
\Rightarrow B = - A\\
\Rightarrow B = - 4x{y^2} - \frac{2}{3}{x^2}y - 4xy - 6\\
c)A + C = - 2xy + 1\\
\Rightarrow C = - A - 2xy + 1\\
\Rightarrow C = B - 2xy + 1\\
\Rightarrow C = - 4x{y^2} - \frac{2}{3}{x^2}y - 4xy - 6 - 2xy + 1\\
\Rightarrow C = - 4x{y^2} - \frac{2}{3}{x^2}y - 6xy - 5
\end{array}$
