已知等差数列
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的前
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项和为
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,且
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,
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.
(1)求
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的通项公式;
(2)设
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,求证:数列
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是等比数列,并求其前
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项和
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(1)
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,
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,解得
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,
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,
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; ………………………………7分
(2)
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,
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,
于是数列
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是以
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为首项,
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为公比的等比数列;
其前
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项的和
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.
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
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
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
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
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