设数列{a n }的前n项和为S n ,a 1 =1,S n =na n -2n(n-1),
1个回答
(Ⅰ)由
得
,
即
,
∴数列{a
n
}是以1为首项,4为公差的等差数列。
(Ⅱ)由(Ⅰ)知
,
∴
。
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