lim(x趋向于3) sin(x-3)/(x*x-9)=lim(x趋向于3) sin（x-3）/[(x-3)(x+3)]
=[lim(x趋向于3) sin（x-3）/(x-3)]*[lim(x趋向于3)1/(x+3) ]
=1*1/6
=1/6