an/an-1=n/n-1
an-1/an-2=n-1/n-2
.
a2/a1=2/1
以上各式相乘
(an/an-1)(an-1/an-2)...(a2/a1)=(n/n-1)(n-1/n-2)...(2/1)
an/a1=n
an=2n