等差數(shù)列中
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與
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的等差中項為5,
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與
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的等差中項為7,則

=
。
解:因為等差數(shù)列中
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與

的等差中項為5,
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與
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的等差中項為7
所以

+

=10,
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,故有2d="4,d=2,"
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,故其通項公式即為
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科目:高中數(shù)學
來源:不詳
題型:解答題
已知數(shù)列

,

滿足:

,當

時,

;對于任意的正整數(shù)

,

.設(shè)

的前

項和為

.
(1)計算

,并求數(shù)列

的通項公式;

(2)求滿足

的

的集合.
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在遞增等差數(shù)列

(

)中,已知

,

是

和

的等比中項.

(1)求數(shù)列

的通項公式;
(2)設(shè)數(shù)列

的前

項和為

,求使


時

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設(shè)等差數(shù)列

的公差d不為0,

,若

是

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
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數(shù)列

的前

項和為

,

,

,
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
的通項公式;
(2)等差數(shù)列

的各項為正,其前

項和為

,且

,又

成等比數(shù)列,求

;
(3)數(shù)列

的前

項和為

,求

.
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(本小題滿分14分)已知等差數(shù)列

,首項為1的等比數(shù)列

的公比為

,且

成等比數(shù)列。
(1)求

的通項公式;
(2)設(shè)

成等差數(shù)列,求k和t的值。
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定義一種運算&,對于

,滿足以下性質(zhì):(1)2&2=1,(2)(

&2=(

&2)+3,則2008&2的數(shù)值為
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等差數(shù)列

中,

則

______________
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來源:不詳
題型:填空題
已知數(shù)列{

}的前n項和

滿足:

,且

=1.那么

=
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