19.一列数a1,a2,a3,…,其中a1=$\frac{1}{2}$,an=$\frac{1}{1-{a}_{n-1}}$(n为不小于2的整数),则a2015=( )
| A. | $\frac{1}{2}$ | B. | 2 | C. | -1 | D. | -2 |
11.已知a+b=4,ab=2,则式子3ab-2a-2b的值等于( )
0 278959 278967 278973 278977 278983 278985 278989 278995 278997 279003 279009 279013 279015 279019 279025 279027 279033 279037 279039 279043 279045 279049 279051 279053 279054 279055 279057 279058 279059 279061 279063 279067 279069 279073 279075 279079 279085 279087 279093 279097 279099 279103 279109 279115 279117 279123 279127 279129 279135 279139 279145 279153 366461
| A. | -10 | B. | 2 | C. | -4 | D. | -2 |