题目

在△ABC中， =5， =4，则AB=（ ） A．9       B．3       C．2       D．1 答案：B【考点】平面向量数量积的运算． 【分析】由=4，得，与=5作和，然后结合向量加法的运算法则求得得答案． 【解答】解：由=4，得， 即， 又=5， ∴﹣=， 即． ∴AB=3． 故选：B．阅读对话，判断正 √ 错×。A: We have a new classroom.B: OK. Let’s go and have a look.A: Look, five fans and four doors.B: And a new computer. It’s cool.A: I like my new classroom.B: Me too.( )（1）We have a new classroom.( )（2）We have two fans.( )（3）We have six lights.( )（4）We have a new computer.( )（5）We like the new classroom.