7.下列算式正确的是( )
| A. | 2$\sqrt{3}$×3$\sqrt{3}$=6$\sqrt{3}$ | B. | $\sqrt{2}$÷$\sqrt{3}$=$\sqrt{5}$ | C. | 5$\sqrt{5}$-2$\sqrt{2}$=3$\sqrt{3}$ | D. | $\sqrt{2}$÷$\sqrt{3}$=$\frac{\sqrt{6}}{3}$ |
3.取一张边长为1的正方形纸片,按如图所示的方法折叠两次,则线段DE的长为( )

| A. | $\sqrt{2}$-1 | B. | $\sqrt{2}$+1 | C. | $\frac{2}{3}$ | D. | $\frac{\sqrt{2}+1}{2}$ |
2.用反证法证明“a≤b“时,应假设( )
| A. | a>b | B. | a<b | C. | a=b | D. | a≥b |
20.一个四边形的四个内角度数之比为1:2:3:3,则这个四边形中,最小的内角为( )
| A. | 30° | B. | 40° | C. | 50° | D. | 60° |
18.
某体育装备公司田径场铺设塑料跑道,中间休息了一段时间,已知跑道面积S(m2)与工作时间t(h)的函数关系如图,则休息后每小时铺设的跑道面积为( )
0 286676 286684 286690 286694 286700 286702 286706 286712 286714 286720 286726 286730 286732 286736 286742 286744 286750 286754 286756 286760 286762 286766 286768 286770 286771 286772 286774 286775 286776 286778 286780 286784 286786 286790 286792 286796 286802 286804 286810 286814 286816 286820 286826 286832 286834 286840 286844 286846 286852 286856 286862 286870 366461
| A. | 40m2 | B. | 50m2 | C. | 60m2 | D. | 70m2 |