=lim(x,y)-(0,0) [(xy+9)-9]/[xy·(根号下(xy+9) +3)]
=lim(x,y)-(0,0) (xy)/[xy·(根号下(xy+9) +3)]
=lim(x,y)-(0,0) 1/[根号下(xy+9) +3]
= 1/6