在各项都为正数的等比数列{a n }中,已知a 3 =4,前三项的和为28.

1个回答

  • (Ⅰ)设公比为q,则有a 3=4,前三项的和为28,

    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.