19.△ABC中,若A=60°,a=$\sqrt{3}$,则△ABC的外接圆半径等于( )
A. | $\frac{\sqrt{3}}{2}$ | B. | 1 | C. | $\sqrt{3}$ | D. | 2 |
18.若向量$\overrightarrow{AB}$=(-2,-3),$\overrightarrow{AC}$=(-4,-7),则$\overrightarrow{BC}$=( )
A. | (-2,-4) | B. | (2,4) | C. | (6,10) | D. | (-6,-10) |
17.12,13,16,21,( ),37.
A. | 25 | B. | 26 | C. | 28 | D. | 31 |
15.在等差数列{an}中,a2+a8=10,则a5=( )
A. | 10 | B. | 5 | C. | 8 | D. | 6 |
14.数列$1\frac{1}{2},2\frac{1}{4},3\frac{1}{8},4\frac{1}{16},…$的通项公式an可以是( )
0 247681 247689 247695 247699 247705 247707 247711 247717 247719 247725 247731 247735 247737 247741 247747 247749 247755 247759 247761 247765 247767 247771 247773 247775 247776 247777 247779 247780 247781 247783 247785 247789 247791 247795 247797 247801 247807 247809 247815 247819 247821 247825 247831 247837 247839 247845 247849 247851 247857 247861 247867 247875 266669
A. | ${a_n}=n+\frac{1}{2^n}$ | B. | ${a_n}=n•\frac{1}{2^n}$ | C. | ${a_n}=n+\frac{1}{{{2^{n-1}}}}$ | D. | ${a_n}=({n-1})+\frac{1}{{{2^{n-1}}}}$ |