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精英家教網 > 高中數學 > 題目詳情

已知是夾角為60°的兩個單位向量,則的夾角的余弦值是(    )

A.B.C.D.

D

解析試題分析:   
,同理 
考點:兩向量夾角
點評:設的夾角為,則

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