(x+y)^2=3,(x-y)^2=5，求xy和x^2+y^2

(x+y)^2=x^2+2xy+y^2=3 (1)
(x-y)^2=x^2-2xy+y^2=5 (2)
(1)-(2)
4xy=-2
所以xy=-1/2

(1)+(2)
2x^2+2y^2=8
所以x^2+y^2=4

(x+y)^2=3,(x-y)^2=5
x^+y^2+2xy=9
x^+y^2-2xy=25
4xy=-16
xy=-4
x^+y^2=17

(x+y)^2=x^2+2xy+y^2=3,
(x-y)^2=x^2-2xy+y^2=5,

两式相加，x^2+y^2=4,

两式相减，xy=-1/2

两式化简后相减，得到4xy＝－2。化简后相加，得到2（x^2+y^2）＝8