$A=(x+y)(x^{2}-xy+y^{2})=4(x+y)^{2}-12xy\geq 4(x+y)^{2}-12.\dfrac{(x+y)^{2}}{4}=16$
$B=(x^{2}+y^{2})^{2}-2x^{2}y^{2}=\left [ (x+y)^{2}-2xy \right ]^{2}-2(xy)^{2}\geq \left [ (x+y)^{2}-2.\dfrac{(x+y)^{2}}{4} \right ]^{2}-2.\left [ \dfrac{(x+y)^{2}}{4} \right ]^{2}=32$
Dấu "=" xảy ra$\Leftrightarrow x=y=2$