问题
解答题
记数列{an}的前n项和为Sn,且a1=1,an+1=2Sn+1.已知数列{bn}满足bn-2=3log3an.
(Ⅰ)求{an}和{bn}的通项公式;
(Ⅱ)设cn=an•bn,求数列{cn}的前n项和Tn.
答案
(Ⅰ)由an+1=2Sn+1,得an=2Sn-1+1,(n≥2)
两式相减,得an+1-an=2an,an+1=3an,(n≥2)
又a2=2S1+1,∴a2=3a1.
所以{an}是首项为1,公比为3的等比数列.
∴an=3n-1.…(4分)
又∵bn=3log3an+2=3log33n-1+2=3(n-1)+2=3n-1.
∴bn=3n-1..…(7分)
(Ⅱ)由(Ⅰ),得cn=(3n-1)×3n-1..…(8分)
∴Tn=2×1+5×31+8×32+…+(3n-4)×3n-2+(3n-1)×3n-1,…(9分)
3Tn=2×3+5×32+8×33+…+(3n-4)×3n-1+(3n-1)×3n,
两式相减,得:-2Tn=2+3×3+3×32+…+3×3n-1-(3n-1)×3n=-
-1 2
×3n,6n-5 2
∴Tn=
+1 4
•3n…(13分)6n-5 4
应改为:-2Tn=2+3×3+3×32+…+3×3n-1-(3n-1)×3n=-
-5 2
×3n,6n-5 2
∴Tn=
+5 4
•3n…(13分)6n-5 4