(本題滿分14分
)設(shè)數(shù)列
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的前
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項(xiàng)和為
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,
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,當(dāng)
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時,
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.
(Ⅰ)若
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,求
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及
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;
(Ⅱ)求

的通項(xiàng)公式.
解:(Ⅰ)在
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中取
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得,
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,
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. ---------2分
由
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得
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相減可得
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,即當(dāng)
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時
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.
可見,

. ----------------4分
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
---------------6分
注:只寫結(jié)果扣1分.
(Ⅱ)由
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得

相減可得
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,即當(dāng)

時

. -----------------9分
其中,

若
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,則由
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知第二項(xiàng)之后是公差為

的等差數(shù)列,但
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,故
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是等差數(shù)列,
若
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,則
若
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,

,則由

可得

于是當(dāng)

時,

是一個公比為

的等比數(shù)列 -----------------12分
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即

(

).

也適合上式,故

的通項(xiàng)公式為

. ------------14分
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