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已知,且的夹角为120°.
求:(1)  ;         (2) ;       (3) .
(1)-1
(2)-5
(3)

试题分析:(1)     ……4分
(2)   ……8分
(3)
点评:主要是考查了向量的数量积的坐标运算,属于基础题。
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