等腰梯形ABCD中，AB∥CD，DC=AD=2，∠A=60°，则=（　　）                  ...

问题详情：

等腰梯形ABCD中，AB∥CD，DC=AD=2，∠A=60°，则=（　　）                   

A．6                                  B．﹣6                              C．﹣3                              D．2

【回答】

B【分析】可画出图形，根据条件即可得到，根据向量减法几何意义即可得到，从而由向量的数量积的运算即可得出的值．                    

【解答】解：如图，根据条件，AB=4，，∠D=120°；                                    

， =；                                                     

∴                                                                

=                                                                                    

=﹣8﹣2+4                                                                                                          

=﹣6．                                                                                                                

故选B．                                                                                                             

【点评】考查等腰梯形的定义，向量减法的几何意义，以及向量的数乘运算，向量的数量积的运算及计算公式．                                                   

知识点：平面向量

题型：选择题