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
-
6n-5
2
×3n

Tn=

1
4
+
6n-5
4
3n…(13分)

应改为:-2Tn=2+3×3+3×32+…+3×3n-1-(3n-1)×3n=-

5
2
-
6n-5
2
×3n

Tn=

5
4
+
6n-5
4
3n…(13分)

单项选择题
单项选择题