(本題12分)等比數(shù)列
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中,已知
(I)求數(shù)列
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的通項公式;
(Ⅱ)若
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分別為等差數(shù)列
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的第3項和第5項,試求數(shù)列
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的通項公式及前
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項和
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。
解:(I)設
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的公比為
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由已知得
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,解得
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(Ⅱ)由(I)得
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,
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,則
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,
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設
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的公差為
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,則有
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解得
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從而
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所以數(shù)列
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的前
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項和
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