设P(y^2/2,y) 则|PA|=[(y^2/2-2/3)^2+y^2]开根号,
[(y^2/2-2/3)^2+y^2]开根号=[(y^2+2/3)^2+12/9]开根号再除以2,当y=0时|PA|最小为2/3.P(0,0)