[    ]

A.2    B.3    C.4    D.5

＝ 

A．2                 B.3      C.4     D.6

 

A(2,-4,-1),B(-1,5,1),C(3,-4,1),令a=,b=,则a+b对应的点为(    )

A.(5,-9,2)        B.(-5,9,-2)             C.(5,9,-2)       D.(5,-9,2)

若,则的值是(    )

A.2               B.3             C.4             D.6

 

若 | a | = 2, | b | = 5, | a +b | = 4，则| a-b |的值为                                                    

A．                      B．3                            C．               D．7