x^2-xy=10,xy-y^2=20,求: x^2-y^2 x^-2xy+y^2

x^2-xy=10,xy-y^2=20,求: x^2-y^2 x^-2xy+y^2

x^2-xy=10
xy-y^2=20
(x^2-xy)+(xy-y^2)=30
x^2-y^2=30
x^2-xy=10
xy-y^2=20
(x^2-xy)-(xy-y^2)=x^-2xy+y^2=-10
x^-2xy+y^2=-10