已知等差数列{an}的前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13成等比数列. (Ⅰ)求数列{an}的通项公式; (Ⅱ)设{bnan}是首项为1,公比为3的等比数列,求数列{bn}的通项公式Tn.

问题描述:

已知等差数列{an}的前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13成等比数列.
(Ⅰ)求数列{an}的通项公式;
(Ⅱ)设{

bn
an
}是首项为1,公比为3的等比数列,求数列{bn}的通项公式Tn

(I)根据题意,可得3a1+3×22d+5a1+4×52d=50(a1+3d)2=a1(a1+12d),a1=3d=2∴an=a1+(n-1)d=3+2(n-1)=2n+1.(II)bnan=3n−1,bn=an•3n−1=(2n+1)•3n-1Tn=3×1+5×3+7×32+…+(2n+1)•3n-1,∴3Tn=...