an等差数列 bn前n项和sn满足sn=3(bn－1)／2 且a2=b1 a5=b2 ⑴求an bn通项 ⑵设tn为数列sn的前n项和,求tn
问题描述：
an等差数列 bn前n项和sn满足sn=3(bn－1)／2 且a2=b1 a5=b2 ⑴求an bn通项 ⑵设tn为数列sn的前n项和,求tn
答
1.Sn=3/2bn-3/2S(n-1)=3/2b(n-1)-3/2bn=Sn-S(n-1)=3/2bn-3/2b(n-1)bn=3b(n-1)所以{bn}是等比数列,公比3而b1=S1=3/2b1-3/2b1=3所以bn=3^na2=b1=3,a5=b2=9d=(a5-a2)/3=2an=2n-12.Sn=3/2*3^n-3/2Tn=9/2(1-3^n)/(1-3)-3/...