(Ⅰ)设公比为q,则有a 3=4,前三项的和为28,
知
a 1 q 2 =4
a 1 (1- q 3 )
1-q =28 ,
解得 a 1 =16,q=
1
2 ,或 a 1 =36,q=-
1
3 .
∵等比数列{a n}各项都为正数,
∴ a 1 =36,q=-
1
3 不合题意,舍去.
∴ a 1 =16,q=
1
2 ,
a n =16× (
1
2 ) n-1 =32× (
1
2 ) n .
(Ⅱ)∵ a n =32× (
1
2 ) n ,
∴b n=log 2a n= log 2 [32×(
1
2 ) n ] =5-n.
S n=b 1+b 2+…+b n=4+3+2+…+(5-n)
=
n(9-n)
2 .
∴
S n
n =
9-n
2 ,
∴
S 1
1 +
S 2
2 +…+
S n
n =
9-1
2 +
9-2
2 +…+
9-n
2
=
9n
2 -
n(n+1)
2
=-(
1
2 n 2 -4n )
= -
1
2 (n-4 ) 2 +8 .
∴n=4时,
S 1
1 +
S 2
2 +…+
S n
n 取最大值8.