解：由方程组
{x+y=7
{xy=12
可得
{x=4
{y=3
或{x=3
{y=4
∵x
所以1/x-1/y=1/3-1/4=1/12
1/x-1/y=(y-x)/xy=(y-x)/12
因为（y-x)^2=(y+x)^2-4xy
所以（y-x)^2=7^2-4*12=49-48=1
所以（y-x)=+1或-1
因为x
x+y=7=3+4
xy=3*4=12
x
1/x-1/y
=(y-x)/(xy)
=(4-3)/12
=1/12
(x+y)²-(x-y)²
=(x+y+x-y)(x+y-x+y)
=4xy
则49-(x-y)²=4×12
(x-y)²=1
x
y-x=1
所以1/x-1/y
=(y-x)/xy
=1/12
(x+y)^2=49
x^2+ 2xy+ y^2=49
x^2+ 2xy+ y^2-4xy=1
x^2-2xy+ y^2=1
(x-y)^2=1
y-x=正负 1 由于x
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