若数列{an}的前n项和为Sn,对任意正整数n都有6Sn=1-2an.(1)求数列{an}的通项公式;(2)令bn=(-1)n-

2025-06-21 14:58:18
推荐回答(1个)
回答1:

(1)∵6Sn=1-2an
∴当n=1时,6a1=1-2a1,解得,a1=

1
8

当n≥2时,6(Sn-Sn-1)=(1-2an)-(1-2an-1),
即an=
1
4
an-1
∴数列{an}是以
1
8
为首项,
1
4
为公比的等比数列,
∴an=
1
8
?(
1
4
n-1=
1
2
?
1
4
n
(2)∵log
1
2
1
2
?(
1
4
)n
=2n+1,
故bn=(-1)n-1?
4(n+1)
log
1
2
an?log
1
2
an+1

=(-1)n-1?
4(n+1)
(2n+1)(2n+3)

∵当n为偶数时,
bn-1+bn=
4n
(2n?1)(2n+1)
-
4(n+1)
(2n+1)(2n+3)

=
4
(2n?1)(2n+3)
=
1
2n?1
-
1
2n+3

故Tn=b1+b2+b3+b4+…+bn-1+bn
=(b1+b2)+(b3+b4)+…+(bn-1+bn
=
1
3
-
1
7
+
1
7
-
1
11
+…+
1
2n?1
-
1
2n+3

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

当n为奇数时,
Tn=b1+b2+b3+b4+…+bn-2+bn-1+bn
=(b1+b2)+(b3+b4)+…+(bn-2+bn-1)+bn
=
1
3
-
1
7
+
1
7
-
1
11
+…+
1
2n?3
-
1
2n+1
+
4(n+1)
(2n+1)(2n+3)

=
1
3
-
1
2n+1
+
4(n+1)
(2n+1)(2n+3)

=
2(n+3)
3(2n+3)

故Tn=
2n
3(2n+3)
,n为偶数
2(n+3)
3(2n+3)
,n为奇数
n∈N*