(n+1){a(n+1)}²=-an*a(n+1)+nan²
(n+1){a(n+1)}²+an*a(n+1)-nan²=0
[a(n+1)+an]*[(n+1)a(n+1)-nan]=0
因为 an>0
(n+1)an+1=nan
an/a(n+1)=(n+1)/n
.
a1/a2=2
把所有的相乘得到
a1/{a(n+1)=n+1
a1=1
a(n+1)=1/(n+1)
an=1/n