已知等差數(shù)列
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的前n項和為
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,
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,
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,
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為等比數(shù)列,且
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,
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,則
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的值為( )
試題分析:在等差數(shù)列
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中,由
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,則
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,
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;由
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,
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,
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,故
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,
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,又?jǐn)?shù)列
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為等比數(shù)列,故
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,
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.
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設(shè)等差數(shù)列
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的前
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項和為
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,已知
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,
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.
(1)求
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;
(2)若從
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中抽取一個公比為
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的等比數(shù)列
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,其中
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,且
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,
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①當(dāng)
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取最小值時,求
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的通項公式;
②若關(guān)于
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的不等式
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有解,試求
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.
(Ⅰ)求數(shù)列
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,數(shù)列
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前n項和為
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,比較
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與2的大�。�
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