x+y=1
所以1/x+2/y
=(1/x+2/y)(x+y)
=1+y/x+2x/y+2
=3+(y/x+2x/y)
y/x>0,2x/y>0
所以y/x+2x/y>=2√(y/x*2x/y)=2√2
所以1/x+2/y最小值=3+2√2