7. 过抛物线的焦点作直线交抛物线于A、B两点，若线段AB中点的横坐标为3，则等于

　 A．10                           B．8                           C．6                           D．4

A. [－1,0]                      B. －∞,0]                 C. [－1,+∞                D. [－1,1

6. 实数、满足 则=的取值范围是

5. 若 ，且, 则实数的值为

Ａ. 1或3                       Ｂ. -3                          C. 1                             Ｄ. 1或 -3

A. 12                             B. 3                             C. 6                             D.

4. 已知两个向量a、b满足ab =－，| a |=4，a和b的夹角为135°，则| b |为

   A. 0.1                                   B. 0.2                          C. 0.3                          Ｄ. 0.4

3. 已知随机变量，若，则等于

2. 正项等比数列中，若，则等于

   A. -16                            B. 10                           C. 16                           D. 256

A.                          B.                         C.                             D.