后项比前项的极限：lim(x^(n+1)/(3^(n+1)+n+1))/(x^(n)/(3^(n)+n)=x/3, 故收敛半径=3当x=±3时，lim(x^(n)/(3^(n)+n))不为0(，级数发散收敛域为(-3,3)
