1,已知数列｛an｝满足：a1=1,an+1=an + 3^n,则an=_____.

题目

1,已知数列｛an｝满足：a1=1,an+1=an + 3^n,则an=_____.
2,已知数列｛an｝的首项a1及公差d都是整数,前n项和为Sn,若a1＞1,a4＞3,S3≤9,设
Bn=1/(n ×an),则使B1+B2+……+Bn＜99/100成立的最大n值为（ ）.
A 97 B 98 C 99 D 100

答案

an为首项为1,公比为3的等比数列前n项和
an=(1+3^n)/2
S3=3*a2 ==>> a2≤3 ==>> an=n+1
Bn=1/(n²+n)=1/n-1/(n+1)
B1+.Bn=1-1/(n+1)＜99/100
==>> n＜99
n=98