x-y=3,xy=9,x+y=?
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x-y=3,xy=9,x+y=?
x-y=3,xy=9,x+y=?
x-y=3,xy=9,x+y=?
(x-y)^2=x^2+y^2-2xy=9
(x+y)^2=x^2+y^2+2xy=9+4xy=45
x+y=正负跟45
(x+y)^2=(x-y)^2+4xy=9+36=45
所以x+y=±3sqrt(5)
【sqrt表示根号】
|x|/x+y/|y|+xy/|xy|=
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