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1.因為x+y=5,x^2+y^2=7所以(x+y)^2=25=x^2+y^2+2xy=7+2xy xy=9 2.因為x+y=4,xy=3所以(x+y)^2=16=x^2+y^2+2xy=x^2+y^2+6所以x^2+y^2=10
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