1的妙用：将1=x+y代入，得x+y/x+2(x+y)/y=1+y/x+2+2x/y=3+y/x+2x/y≥3+2√2即最小值为3+2√2
1/x+2/y=(1\x+2/y)(x+y)=3+y/x+2x/y>=3+2根号2
