=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
