(本題滿分14分)設(shè)等差數(shù)列
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的前
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項(xiàng)和為
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,已知
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,
(1)求
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的通項(xiàng)公式;
(2)若
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,求
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。
(1)
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;
(2)
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本試題主要是考查了等差數(shù)列的前n項(xiàng)和公式和數(shù)列的通項(xiàng)公式之間的關(guān)系的運(yùn)用。
(1)因?yàn)榈炔顢?shù)列
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的前
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項(xiàng)和為
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,根據(jù)
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,
設(shè)出首項(xiàng)和公差,可以求
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的通項(xiàng)公式;
(2)在第一問的基礎(chǔ)上可知
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,得到n的值。
解:(1)由
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,
得方程組
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……..4分
解得,
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,……..6分
故
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……..7分
(2)由
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……..10分
得方程
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,解得
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或
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(舍去)
故
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……..14分
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.
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
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=
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