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春芽
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设公比为q,∵S 2n -S n =6480>S n ,
∴q>1.
又由a n >0,则最大项是a n =a 1 q n-1 =54;①
又S n =
a 1 (1- q n )
1-q =80,②
S 2n =
a 1 (1- q 2n )
1-q =6560,③
由①②③解得a 1 =2,q=3,则
(1)前100项之和S 100 =
2( 3 100 -1)
3-1 =3 100 -1.
(2)通项公式为a n =2•3 n-1 .
1年前
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