A
xy≤[(x+y)/2]^2
xy-(x+y)=1
xy=(x+y)+1
x+y+1≤[(x+y)/2]^2
x,y属于R*
x+y>=(2根号2)+2
B,D错误原因如下：
x+y≥2根号xy
xy-(x+y)=1
x+y=xy-1
xy-1≥2根号xy
xy≥3+2根号2