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春芽
共回答了12个问题采纳率:83.3% 举报
(1)∵正项等比数列{a n }中, a 3 2 =
1
4 a 2 •a 6 =
1
4 a 4 2 ,
∴q 2 =
a 4 2
a 3 2 =4,q>0,
∴q=2;
又2a 1 +3a 2 =8,即2a 1 +3a 1 q=8,
∴a 1 =1.
∴S n =
1×(1 -2 n )
1-2 =2 n -1.
(2)∵b n =
1
log 2 ( S n +1). log 2 ( S n+1 +1)
=
1
log 2 (2 n -1+1). log 2 (2 n+1 -1+1)
=
1
log 2 2 n . log 2 2 n+1
=
1
n(n+1)
=
1
n -
1
n+1 ,
∴T n =b 1 +b 2 +…+b n
=(1-
1
2 )+(
1
2 -
1
3 )+…+(
1
n -
1
n+1 )
=1-
1
n+1
=
n
n+1 .
1年前
1