已知直線
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平面
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,直線
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平面
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,給出下列命題,其中正確的是( )
①
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②
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③
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④
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試題分析:因為
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,直線
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平面
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,所以直線
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平面
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.又因為,直線
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平面
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.所以
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.所以①式正確.所以可以排除下列B,C選項.若
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,直線
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平面
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,直線
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
平面
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,則
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與
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可以有平行、異面、相交三種位置關系.所以②不正確.故選A.
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如圖,在正方體

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(1)求證:
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;
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
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
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
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
,
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為

的中點,

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
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
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⊥平面

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設
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α,
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
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在正方體
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中,過對角線

的一個平面交棱
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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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”是“
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,
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已知
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是兩條不重合的直線,

是兩個不重合的平面,給出下列命題:
①若

,

,且

,則

;
②若
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,
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,且

,則

;
③若

,
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,且
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,則

;
④若

,

,且
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,則

.
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