已知x^2-xy=7,xy-y^2=4,则x^2-y^2=

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已知x^2-xy=7,xy-y^2=4,则x^2-y^2=
已知x^2-xy=7,xy-y^2=4,则x^2-y^2=

已知x^2-xy=7,xy-y^2=4,则x^2-y^2=
x^2-y^2
=x²-xy+xy-y²
=(x²-xy)+(xy-y²)
=7+4
=11

x^2-xy=7,xy-y^2=4
x^2-y^2
=x²-xy+xy-y
=7+4
=11

11数学怎么学的