数列{an},前n项和Sn,满足a1=12,Sn+2an+1=1(n∈N*)(1)求数列{an}的通项公式;(2)求数列{nSn}前n项和Tn.-数学

题目简介

数列{an},前n项和Sn,满足a1=12,Sn+2an+1=1(n∈N*)(1)求数列{an}的通项公式;(2)求数列{nSn}前n项和Tn.-数学

题目详情

数列{an},前n项和Sn,满足a1=
1
2
Sn+2an+1=1(n∈N*)
(1)求数列{an}的通项公式;
(2)求数列{nSn}前n项和Tn
题型:解答题难度:中档来源:不详

答案

(1)∵Sn+2an+1=1(n∈N*)
∴Sn-1+2an=1(n≥2)
两式相减可得,Sn-Sn-1+2an+1-2an=0
即2an+1=an
an+1
an
=class="stub"1
2

a1=class="stub"1
2

∴数列{an}是以class="stub"1
2
为首项以class="stub"1
2
为公比的等比数列
an=class="stub"1
2
•(class="stub"1
2
)n-1
=(class="stub"1
2
)n

(2):∵Sn+2an+1=1(n∈N*)
Sn+2•(class="stub"1
2
)n+1=1

Sn=1-(class="stub"1
2
)n

∴nSn=n-n•(class="stub"1
2
)n

Sn=1•class="stub"1
2
+2•(class="stub"1
2
)2+…+n•(class="stub"1
2
)n

class="stub"1
2
Sn
=(class="stub"1
2
)2+2•(class="stub"1
2
)3+…+(n-1)•(class="stub"1
2
)n+n•(class="stub"1
2
)n+1

两式相减可得,class="stub"1
2
Sn
=class="stub"1
2
+(class="stub"1
2
)2+…+(class="stub"1
2
)n-n•(class="stub"1
2
)n+1

=
class="stub"1
2
(1-class="stub"1
2n
)
1-class="stub"1
2
-n•(class="stub"1
2
)n+1

∴Sn=2-class="stub"1
2n-1
-class="stub"n
2n
=2-class="stub"2+n
2n

Tn=1-1•class="stub"1
2
+2-2•(class="stub"1
2
)2+…+n-n•(class="stub"1
2
)n

=(1+2+3+…+n)-[1•class="stub"1
2
+2•(class="stub"1
2
)2+…+n•(class="stub"1
2
)n]

=
n(n+1)
2
-2+class="stub"2+n
2n

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