已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=4,（x-y）^2=3,则x^2+y^2=?若x^2+y^2=3,xy=1,则x-y=?

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已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=4,（x-y）^2=3,则x^2+y^2=?若x^2+y^2=3,xy=1,则x-y=?
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已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=4,（x-y）^2=3,则x^2+y^2=?若x^2+y^2=3,xy=1,则x-y=?
已知（x+y)^2=9,x^2+y^2=7,则xy=?
已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=4,（x-y）^2=3,则x^2+y^2=?若x^2+y^2=3,xy=1,则x-y=?

已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=9,x^2+y^2=7,则xy=?已知（x+y)^2=4,（x-y）^2=3,则x^2+y^2=?若x^2+y^2=3,xy=1,则x-y=?
(1)2xy=(x+y)^2-(x^2+y^2)=9-7=2 xy=1
(2)2(x^2+y^2)=(x+y)^2+(x-y)^2=4+3=7 x^2+y^2=7/2
(3)(x-y)^2=x^2+y^2-2x=3-2=1 则x-y=1或-1

（x+y）^2=x^2+y^2+2xy=7+2xy=9,所以xy=1.

（x+y)^2=x^2+y^2+2xy=9
xy=(9-7)/2=1
（x+y)^2=4,（x-y）^2=3，
相加：
2(x^2+y^2)=7
则x^2+y^2=7/2
(x-y)^2=x^2+y^2-2xy=1
x-y=1或-1

1,7/2，±1

1.答案为1
2.答案为2分之7
3.答案为正负1
刚发现，这号不是我的，你得好好读书了

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