3.甲、乙、丙、丁四人站一排照相,其中甲、乙不相邻的站法共有n种,则($\root{3}{x}$-$\frac{1}{2x}$)n展开式的常数项为( )
| A. | -$\frac{55}{2}$ | B. | $\frac{55}{2}$ | C. | -55 | D. | 55 |
20.已知{an}为等比数列,a1=3,且4a1,2a2,a3成等差数列,则a3+a5等于( )
| A. | 189 | B. | 72 | C. | 60 | D. | 33 |
18.已知集合A={x|-1<x<3},B={x|y=$\sqrt{1-x}$},则A∩B=( )
0 227743 227751 227757 227761 227767 227769 227773 227779 227781 227787 227793 227797 227799 227803 227809 227811 227817 227821 227823 227827 227829 227833 227835 227837 227838 227839 227841 227842 227843 227845 227847 227851 227853 227857 227859 227863 227869 227871 227877 227881 227883 227887 227893 227899 227901 227907 227911 227913 227919 227923 227929 227937 266669
| A. | {x|1≤x<3} | B. | {x|x<1} | C. | {x|-1<x≤1} | D. | {x|-1<x<1} |