2x+y =
(1/x + 9/y)(2x+y)
=2+(y/x)+(18x/y)+9
=11+(y/x)+(18x/y)
≥11+ 2√[(y/x)·(18x/y)]
=11+2√18
=11+6√2
当且仅当 y/x = 18x/y等号成立