问题 解答题

已知数列{an}的前n项和为Sn,且an是Sn与2的等差中项,而数列{bn}的首项为1,bn+1-bn-2=0.

(1)求a1和a2的值;

(2)求数列{an},{bn}的通项an和bn

(3)设cn=an•bn,求数列{cn}的前n项和Tn

答案

(1)∵an是Sn与2的等差中项,

∴Sn=2an-2,∴a1=S1=2a1-2,解得a1=2,a1+a2=S2=2a2-2,解得a2=4;

(2)∵Sn=2an-2①,∴Sn-1=2an-1-2(n≥2)②,

①-②得:an=2an-2an-1,即an=2an-1(n≥2,n∈N*)

∵a1≠0,∴

an
an-1
=2,(n≥2,n∈N*),即数列{an}是等比数列.

∵a1=2,∴an=a1qn-1=2×2n-1=2n

由已知得bn+1-bn=2,即数列{bn}是等差数列,

又b1=1,∴bn=b1+(n-1)d=1+2(n-1)=2n-1;

(3)由cn=an•bn=(2n-1)2n

Tn=a1b1+a2b2+…+anbn=1×2+3×22+5×23+…+(2n-1)2n③,

2Tn=1×22+3×23+…+(2n-3)2n+(2n-1)2n+1④,

③-④得:-Tn=1×2+(2×22+2×23+…2×2n)-(2n-1)2n+1

即:-Tn=1×2+(23+24+…2n+1)-(2n-1)2n+1=2+

23(1-2n-1)
1-2
-(2n-1)2n+1

Tn=(2n-3)2n+1+6

单项选择题
单项选择题