`~rai~`
\(a)A=5xy^2+xy-xy^2-\dfrac{1}{3}x^2y+2xy+x^2y+xy+6\\\quad\quad=(5xy^2-xy^2)+\left(x^2y-\dfrac{1}{3}x^2y\right)+(xy+2xy+xy)+6\\\quad\quad=4xy^2+\dfrac{2}{3}x^2y+4xy+6.\\\text{Bậc của đa thức:1+2=3.}\\b)A+B=0\\\Leftrightarrow B=-A\\\Leftrightarrow B=-\left(4xy^2+\dfrac{2}{3}x^2y+4xy+6.\right)\\\Leftrightarrow B=-4xy^2-\dfrac{2}{3}x^2y-4xy-6.\\\text{Vậy B=}-4xy^2-\dfrac{2}{3}x^2y-4xy-6.\\c)A+C=-2xy+1\\\Leftrightarrow C=-2xy+1-A\\\Leftrightarrow C=-2xy+1-\left(4xy^2+\dfrac{2}{3}x^2y+4xy+6\right)\\\Leftrightarrow C=-2xy+1-4xy^2-\dfrac{2}{3}x^2y-4xy-6\\\Leftrightarrow C=-4xy^2-\dfrac{2}{3}x^2y-(4xy+2xy)+(1-6)\\\Leftrightarrow C=-4xy^2-\dfrac{2}{3}x^2y-6xy-5.\\\text{Vậy C=}-4xy^2-\dfrac{2}{3}x^2y-6xy-5.\)
