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已知数列{an}为等差数列,Sn为其前n项和,a1+a5=6,S9=63.
(1)求数列{an}的通项公式an及前n项和Sn
(2)数列{bn}满足:对∀n∈N*bn2an,求数列{an•bn}的前n项和Tn
人气:210 ℃ 时间:2020-01-30 00:10:22
解答
(1)∵S9=63,∴9a5=63,解得a5=7.
∵a1+a5=6,∴a1=-1,
∴d=
a5a1
4
=2

∴an=2n-3,Sn=n2−2n
(2)∵an=2n-3,bn=2an
bn=22n−3
∴an•bn=(2n-3)•22n-3
Tn=−1•2−1+1•21+3•23+5•25+…+(2n-3)•22n-3
4Tn=-1×21+1•23+3•25+…+(2n-5)•22n-3+(2n-3)•22n-1
两式相减,得:-3Tn=-
1
2
+2(2+23+25+…+22n−3)−(2n−3)•22n−1

=-
1
2
+2•
2(1−22(n−1))
1−22
−(2n−3)•22n−1

=
(11−6n)•22n−11
6

Tn
(6n−11)•22n+11
18
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