题型：问答题 简答题 已知两个平面垂直，下列命题①一个平面内的已知直线必垂直于另一个平面内的任意一条直线 ②一个平面内的已知直线必垂直于另一个平面内的无数条直线 ③一个平面内的任一条直线必垂直于另一个平面 ④过一个平面内任意一点作交线的垂线，则此垂线必垂直于另一个平面其中正确命题的个数是（ ）A.0B.1C.2D.3 查看答案
题型：问答题 简答题 阅读理解。 Our teeth are very important, but how to take care of the teeth? Most people don't know. To help more people pay attention to their teeth heath, China sets up the first Love Teeth Day on September 20, 1989. Every year on this day, there are many different kinds of activities to tell people how to take careof their teeth. After more than 20 years, now Chinese people care more about their teeth. Many people know howto brush their teeth in right way. They can take good care of. If you want to be a good kid, please do it by the following ways.1. Respect your parents. 2. Clean your room, do the dishes, take out the trash (垃圾), and so on. 3. Be polite to people. Say goodbye when you go out. Say thank you when others help you. 4. Study hard at school. 5. Ask questions in class. 6. Learn to smile. This will show your parent that you are enjoying life. 1. When is Love Teeth Day? [ ]A. September 20, 1989.B. October 1, 1989. C. October 20, 1989. D. September 10, 1989. 2. How many Love Teeth Days do we have by 2010?[ ]A. more than 10B. more than 15C. more than 18D. more than 203. A good kid should (应该) be_____ to people.[ ]A. bad B. polite C. bored D. boring 4. _____ will show your parents that you are enjoy life.[ ]A. Smiling B. Talking C. Studying D. Crying 5. As a good kid, you shouldn't _____. [ ]A. clean your room B. study hardC. look for any trouble D. thank others 查看答案
题型：问答题 简答题 如图所示，一位宇航员站在某质量分布均匀的星球表面的一斜坡上的A 点，沿水平方向以初速度v0抛出一个小球，测得小球经时间t落到斜坡上另一点B，斜坡的倾角为α，已知该星球的半径为R。求： （1）该星球表面的重力加速度；（2）该星球的第一宇宙速度。 查看答案
