证明：
原式=[（x-y)^2+2xy]/(x-y)
=[（x-y)^2+2]/(x-y)
=(x-y)+2/(x-y)
因为x＞y＞0 ∴x-y＞0
所以(x-y)+2/(x-y) >=2√[（x-y)*2/(x-y)]=2√2