问题
解答题
已知数列{an}的前n项和为Sn,且Sn=1-an(n∈N*). (1)试求{an}的通项公式; (2)若bn=
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答案
(1)n=1时,a1=1-a1,a1=
1 |
2 |
∵Sn=1-an(n∈N*)①,∴Sn+1=1-an+1②,
②-①得an+1=-an+1+a n,∴an+1=
1 |
2 |
∴数列{an}是首项为a1=
1 |
2 |
1 |
2 |
∴an=
1 |
2 |
1 |
2 |
1 |
2 |
(2)bn=
n |
an |
∴Tn=1×2+2×22+3×23+…+n×2n,③
2Tn=1×22+2×23+3+3×23+…+n×2n+1,④
③-④得,-Tn=2+22+23+…+2n-n×2 n+1③,
=
2(1-2n) |
1-2 |
整理得Tn=(n-1)2 n+1+2