问题
解答题
已知数列{ an}的前n项和为Sn,且Sn=2an-l;数列{bn}满足bn-1-bn=bnbn-1(n≥2,n∈N*)b1=1. (Ⅰ)求数列{an},{bn}的通项公式; (Ⅱ)求数列{
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答案
(Ⅰ)当n=1时,a1=S1=2a1-1,解得a1=1.
又当n≥2时,an=Sn-Sn-1=2an-1-(2an-1-1)=2an-2an-1,
∴an=2an-1(n≥2).
∴数列{an}是首项为1,公比为2的等比数列.
∴an=1×2n-1=2n-1(n∈N*).
由bn-1-bn=bnbn-1,得
-1 bn
=1.1 bn-1
又b1=1,所以数列{
}是首项为1 bn
=1 b1
=1,公差为1的等差数列.1 1
∴
=1+(n-1)×1=n.1 bn
∴bn=
.1 n
(Ⅱ)由(Ⅰ)可得:
=n•2n-1,an bn
∴Tn=1×20+2×21+…+n•2n-1,
2Tn=1×21+2×22+…+(n-1)•2n-1+n•2n,.
两式相减,得-Tn=1+21+22+…+2n-1-n•2n=
-n•2n=2n-1-n•2n.1×(2n-1) 2-1
∴Tn=(n-1)•2n+1.