x+y=5,
(x+y)^2=25,
x^2+y^2+2xy=25
x-y=1,
(x-y)^2=1,
x^2+y^2-2xy=1
xy=6
x^2+y^2=13
知x+y=5,x-y=1
x²+y²+2xy = 25 ①
x²+y²-2xy = 1 ②
①- ② 4xy= 25-1 = 24
xy = 24 / 4 = 6
x^2+y^2-xy
=x²+y²+2xy -2xy
= 25 - 12
= 13
解:解方程组x+y=5
x-y=1
得到 x=3
y=2
则xy=6
x^2+y^2-xy=9+4-6=7
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