x^2+xy=2 y^2+xy=7,则2（x-y)(x+y)=

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x^2+xy=2y^2+xy=7,则2（x-y)(x+y)=x^2+xy=2y^2+xy=7,则2（x-y)(x+y)=x^2+xy=2y^2+xy=7,则2（x-y)(x+y)=x^2+xy=2--

x^2+xy=2 y^2+xy=7,则2（x-y)(x+y)=
x^2+xy=2 y^2+xy=7,则2（x-y)(x+y)=

x^2+xy=2 y^2+xy=7,则2（x-y)(x+y)=
x^2+xy=2--------1式
y^2+xy=7---------2式
用1式减去2式有x^2-y^2=（x-y)(x+y）=-5

x^2+xy=2
y^2+xy=7
两式相减得
x^2-y^2=-5
(x+y)(x-y)=-5
所以原式=2(x-y)(x+y)=-10

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