(本小题满分14分)
已知数列
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满足
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;
(1)证明:数列
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是等比数列,并求出数列
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的通项公式;
(2)若
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求数列
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的前
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项和为
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;
(3)令
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,数列
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的前
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项和为
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,求证:
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.
(1)
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(2)
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(3)略
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
,
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分别是等差、等比数列,且
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,
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,
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.
①求数列
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,

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②设

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
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
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
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
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
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