an+1=（n+1）an/n得an/an-1=n/(n-1)(n≥2)an-1/an-2=(n-1)/(n-2)an-2/an-3=(n-2)/(n-3).a2/a1=2/1上面的n-1个式子,相乘得an/a1=n/(n-1)*(n-1)/(n-2)*(n-2)/(n-3)*.*2/1=n∴an=na1=2n