等差數(shù)列
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的公差
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,且
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,僅當
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時,數(shù)列
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的前
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項和
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取得最大值,則首項
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的取值范圍是
解:因為已知等差數(shù)列當n=9時,前n項和取得最大值,可知要是滿足題意,結(jié)合數(shù)列的通項公式可知,首項的范圍由公差的范圍解得為C.
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(1)求
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