因为x^2+xy=3,设为一式，xy+y^2=-2,设为二式，一式减二式得：x^2-y^2=5,因为：2x^2-xy-3y^2=(2x^2-2y^2)-(xy+y^2)=2*5-(-2)=12
2x平方-xy-3y平方
=2(x的平方+xy)-3(xy+y的平方)
=2*3-3*(-2)
=6+6
=12
x^2+xy=3,
x(x+y)=3
xy+y^2=-2,
y(x+y)=-2
x/y=3/(-2)
-2x=3y
2x^2-xy-3y^2
=(2x-3y)(x+y)
=4x(x+y)
=12
x^2+xy=3,(1)
xy+y^2=-2,(2),
(1)*2-(2)*3
2(x^2+xy)-3(xy-y^2)
=2x^2-xy-3y^2
=6+6=12
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