问题
解答题
已知等比数列{an}中,前n项之和Sn=P•3n-
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
①a1=S1=3p-
,a2=S2-S1=6p,a3=S3-S2=18p∵an等比数列,∴a22=a1a3,∴p=3 2 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