在学习《圆》这一章时,老师给同学们布置了一道尺规作图题:
尺规作图:过圆外一点作图的切线。
已知:P为圆O外一点。
求作:经过点P的圆O的切线。
小敏的作法如下:
①连接OP,作线段OP的垂直平分线MN交OP于点C;
②以点C为圆心,CO的长为半径作圆交圆O于A、B两点;
③作直线PA、PB,所以直线PA、PB就是所求作的切线。
老师认为小敏的作法正确.
请回答:连接OA,OB后,可证∠OAP=∠OBP=90°,其依据是____;由此可证明直线PA,PB都是⊙O的切线,其依据是____
These days computer games are becoming more and more popular in many cities and towns. A lot of small shops along the 1have changed into computer game houses in order to get more 2. These places are always crowded with people.In the computer game houses, people 3a lot of money matching on the machine. It’s 4for one to beat a computer, but one can do well after trying again and again. People want to 5when they play computer games. The more they 6, the more they want to win, and at last they even can’t 7without it.The result(结果) is that some people don’t want to 8and they play in computer game houses for hours and hours. For school boys, things are 9. They don’t want to have 10. When school is over, they rush to the computer game houses near their 11. Some of them can get enough money from their 12. Some of them are not 13 enough to get the money. So they have to steal(偷) or rob(抢) others’ money and become 14.Computer game addiction(上瘾) is a(n) 15problem in our life. Something has to be done to stop it.
①洋务运动的核心是引进西方先进的科学技术和大机器生产
②洋务运动是清政府自上而下的自救运动
③洋务运动使中国开始进入世界资本主义工业化潮流
④洋务运动中出现了中国最早的资本主义性质的企业