A=1/2xy^2+2/3x^2y+xy+xy^2-1/3x^2y tại x=1/2 và y =-1

$\displaystyle \begin{array}{{>{\displaystyle}l}} \frac{1}{2} xy^{2} +\frac{2}{3} x^{2} y+xy+xy^{2} -\frac{1}{3} x^{2} y\\ =\frac{3}{2} xy^{2} +\frac{1}{3} x^{2} y+xy \end{array}$ thay $\displaystyle x=\frac{1}{2} ;y=-1$ vào ta có $\displaystyle \begin{array}{{>{\displaystyle}l}} \frac{3}{2} .\frac{1}{2} .( -1)^{2} +\frac{1}{3} .\left(\frac{1}{2}\right)^{2} .( -1) +\frac{1}{2} .( -1)\\ =\frac{3}{4} -\frac{1}{12} -\frac{1}{2}\\ =\frac{9}{12} -\frac{1}{12} -\frac{6}{12}\\ =\frac{1}{6} \end{array}$