1)设首项为a1,公差为d
a2+a3=7=2a1+3d,
a4+a5+a6=18=3a1+12d
得 a1=2 ,d=1
an=1+n (n为大于0的整数）
2) Sn=na1+n(n-1)d/2
=2n+n^2/2-n/2
=n(n+3)/2
3)1/Sn=2/[n(n+3)]
=2/3[1/n-1/(n+3)]
所以S3+S6+.+S3n
=2/3[1/3-1/6+1/6-1/9+.+1/3(n-1)-1/3n]
= 2[1/3-1/3n]/3
=2(n-1)/9n