正数x、y满足正数x、y满足x2-y2=2xy,
x2-y2=2xy
x2-2xy+y2-2y2=0
(x-y)2=2y2
x-y=√2*y
x=(√2+1)y
（x—y）/（x y）==√2*y/(√2+2)y
=√2/(√2+2)
=√2(2-√2)/(4-2)
=√2-1