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

問題詳情：

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

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

【回答】

B【分析】可畫出圖形，根據條件即可得到，根據向量減法幾何意義即可得到，從而由向量的數量積的運算即可得出的值．                    

【解答】解：如圖，根據條件，AB=4，，∠D=120°；                                    

， =；                                                     

∴                                                                

=                                                                                    

=﹣8﹣2+4                                                                                                          

=﹣6．                                                                                                                

故選B．                                                                                                             

【點評】考查等腰梯形的定義，向量減法的幾何意義，以及向量的數乘運算，向量的數量積的運算及計算公式．                                                   

知識點：平面向量

題型：選擇題