x→0 lim sin3x/tan5x =lim sin3x*cos5x / sin5x =lim sin3x/3x * 3x/5x * 5x/sin5x * cos5x =lim sin3x/3x * lim 3x/5x * lim 5x/sin5x * lim cos5x 根据重要的极限:lim(x→0) sinx/x=1 =lim 3/5 * lim cos5x =3/5 * 1 =3/5 有不懂欢迎追问