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        1. 已知向量,且,則m等于(   )
          A.2B.C.D.
          A

          試題分析:因?yàn),向?img src="http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012413093561.png" style="vertical-align:middle;" />,,且,所以,·=-2+m=0,m=2,故選A。
          點(diǎn)評(píng):簡(jiǎn)單題,兩向量垂直,兩向量的數(shù)量積為0.
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