(本小題滿分12分)
已知等差數(shù)列
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的首項
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前
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項和記為
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,求
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取何值時,

取得最大值,并求出最大值.
由等差數(shù)列
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的首項

,設(shè)公差為
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,根據(jù)等差數(shù)列的前
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項和公式可求出
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,所以
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,
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,當(dāng)
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時,
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.所以當(dāng)
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或
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時,
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取得最大值,最大值是

.
∵
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∴
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………………………………………3分
∴
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∴
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………………………………6分
即,當(dāng)

時,
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
∴當(dāng)
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或
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時,
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取得最大值,最大值是
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………12分
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
的首項

,前

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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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
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
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
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
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
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
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
,前

項和

滿足條件

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
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
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
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
項和

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
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
,
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,
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
等于( )
A

B

C

D

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