6．已知直线与圆相切，则实数的值是

A．1                             B．2                            C．4                   D．

5．已知向量a?b=，| a | =4，a和b的夹角为，则| b |为

A．                           B．                         C．                          D．

4．双曲线的离心率为

3．设是等比数列，若，则等于

   A．6                             B．8                            C．9                            D．16

A．                             B．                        C．                      D．

2．化简的结果是

A．                        B．                      C．               D．

1．已知全集，集合，，则等于