12.“$α=\frac{π}{2}$”是“cos2α=-1”的( )
| A. | 充分而不必要条件 | B. | 必要而不充分条件 | ||
| C. | 充分必要条件 | D. | 既不充分也不必要条件 |
10.执行如图所示的程序框图,则输出的结果是( )

| A. | $\frac{22}{23}$ | B. | $\frac{21}{22}$ | C. | $\frac{20}{21}$ | D. | $\frac{19}{20}$ |
9.已知椭圆$\frac{x^2}{9}+\frac{y^2}{b^2}=1$(0<b<3),左右焦点分别为F1,F2,过F1的直线交椭圆于A,B两点,若|AF2|+|BF2|的最大值为8,则b的值是( )
0 226737 226745 226751 226755 226761 226763 226767 226773 226775 226781 226787 226791 226793 226797 226803 226805 226811 226815 226817 226821 226823 226827 226829 226831 226832 226833 226835 226836 226837 226839 226841 226845 226847 226851 226853 226857 226863 226865 226871 226875 226877 226881 226887 226893 226895 226901 226905 226907 226913 226917 226923 226931 266669
| A. | $2\sqrt{2}$ | B. | $\sqrt{2}$ | C. | $\sqrt{3}$ | D. | $\sqrt{6}$ |