设x=y+z(z>0)
则原式=2(y+z)+1/(z^2)=2y+2z+1/(z^2)=2y+z+z+1/(z^2)
而(z+z+1/(z^2))/3》1(三项均值)
故原式=2y+z+z+1/(z^2)》2y+3