$$0.5 { x }^{ 2 } +2 { y }^{ 2 } -0.8xy,0.8 { x }^{ 2 } +4y+0.8xy$$
$-\frac{4xy}{5}+\frac{x^{2}}{2}+2y^{2},\frac{4\left(x^{2}+xy+5y\right)}{5}$
