x>=4
x/y=x-y
x=(x-y)y
x=xy-y2
y2=x(y-1)
x=y2/(y-1)
设y-1=t 因为y>1 所以t>0
故x=(t2+2t+1)/t
x=t+1/t+2>=2根号1+2
x>=4