题目

已知向量 ，则___________. 答案：4【分析】 已知模平方求得 ，再把待求模平方转化为数量积的运算即可得． 【详解】 由已知 ， ， 所以 ， 所以 ． 故答案为： 4．What made him so late for the meeting? [　　] A．Because he had just heard from his parents . B．Because he had just lost his key to the car . C．Losing his key to the car. D．He just heard a letter from his parents .