问题 解答题
已知等比数列{an}中,前n项之和Sn=P•3n-
3
2
(P∈R).
①求P的值.
②求数列{an}的通项公式.
③若数列{bn}满足bn=anlog3an,求和Tn=b1+b2+∧+bn
答案

a1=S1=3p-

3
2
a2=S2-S1=6p,a3=S3-S2=18p∵an等比数列,∴a22=a1a3,∴p=
3
2

②由①知,a1=3,q=3,∴an=3n

③bn=anlog3an=n•3n,Tn=1×3+2×32++n•3n(1)3Tn=1×32+2×33++n•3n+1(2)

(1)-(2)得:-2Tn=3+32++3n-n•3n+1,∴Tn=

(2n-1)•3n+1+3
4

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