已知数列an的前n项和为sn=2n/n+1,则1∕a8等于?

问题描述:

已知数列an的前n项和为sn=2n/n+1,则1∕a8等于?

由sn=2n/n+1得 sn-1=2(n-1)/n
an=sn-sn-1
∴an=2/[n(n+1)]
a1=s1=1满足an=2/[n(n+1)]
∴1/a8=36