試題分析:因為,
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,且
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與
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的夾角為鈍角,所以
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·
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<0,
且
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π,
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,
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,但,
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時,
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=π,故答案為
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。
點評:中檔題, 對于平面向量
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,
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。
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如圖,平面內(nèi)有三個向量

,其中
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與

的夾角為
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,

與

的夾角為
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,且
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,若
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,則( )

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科目:高中數(shù)學(xué)
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在平行四邊形ABCD中,E、F分別是BC、CD的中點,DE交AF于H,記
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、
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分別為
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,則
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=( )
A.
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B.
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C.
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D.
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平面內(nèi)給定兩個向量
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(1)求
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;
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