已知x-y=3,xy=2,则x+y=

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已知x-y=3,xy=2,则x+y=已知x-y=3,xy=2,则x+y=已知x-y=3,xy=2,则x+y=(x-y)²=9x²+y²-2xy+4xy=9+4*2=17x

已知x-y=3,xy=2,则x+y=
已知x-y=3,xy=2,则x+y=

已知x-y=3,xy=2,则x+y=
(x-y)²=9
x²+y²-2xy+4xy=9+4*2=17
x+y=±√17

(x+y)^2=(x-y)^2+4xy=3^2+8=17
所以x+y=√17或-√17

已知x-y=3,xy=2,则x+y=
(x-y)²=(x+y)²-4xy=(x+y)²-8=9
故(x+y)²=17，∴x+y=±√17.

(x+y)^2=(x-y)^2+4xy=3^2+4*2=17

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