已知等差數(shù)列{a
n}的公差為正數(shù),且a
3·a
7=-12,a
4+a
6=-4,則S
20為
因?yàn)?img src="http://thumb.zyjl.cn/pic2/upload/papers/20140823/20140823201439841456.png" style="vertical-align:middle;" />是等差數(shù)列,所以
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,從而可得
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是方程

的兩根,解得
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或

。因?yàn)榈炔顢?shù)列

的公差為正數(shù),所以
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,則
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,所以
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,
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,所以
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,故選A
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來源:不詳
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已知
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是遞增的等差數(shù)列,滿足
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(1)求數(shù)列
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對
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均有

…+

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
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
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
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
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
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
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
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
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
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