(x^2+y^2)/(x-y)=[(x-y)^2+2xy]/(x-y)=(x-y)+6/(x-y)>=2√[(x-y)*6/(x-y)]=2√6[(x-y)^2+2xy]/(x-y)=(x-y)+6/(x-y) 为什么（x-y)^2 变成了 (x-y)？ 那个^2跑哪去了？[(x-y)^2+2xy]/(x-y)=(x-y)^2/(x-y)+2xy/(x-y)=(x-y)+6/(x-y)