3.
已知三棱锥A-BCD的各棱长都相等,E为BC中点,则异面直线AB与DE所成角的余弦值为( )
| A. | $\frac{5\sqrt{3}}{6}$ | B. | $\frac{\sqrt{3}}{6}$ | C. | $\frac{\sqrt{33}}{6}$ | D. | $\sqrt{11}$ |
18.从一批含有11只正品,2只次品的产品中,不放回地抽取3次,每次抽取1只,设抽得次品数为X,则E(5X+1)的值为( )
0 240543 240551 240557 240561 240567 240569 240573 240579 240581 240587 240593 240597 240599 240603 240609 240611 240617 240621 240623 240627 240629 240633 240635 240637 240638 240639 240641 240642 240643 240645 240647 240651 240653 240657 240659 240663 240669 240671 240677 240681 240683 240687 240693 240699 240701 240707 240711 240713 240719 240723 240729 240737 266669
| A. | $\frac{43}{13}$ | B. | $\frac{42}{13}$ | C. | $\frac{12}{13}$ | D. | $\frac{6}{13}$ |