题目

已知点是抛物线上的一点，过作直线的垂线，垂足为，直线经过原点，由上的一点向圆引两条切线，分别切圆于，两点，且为直角三角形，则的最小值是______. 答案：【答案】.【解析】根据抛物线的定义得到 ，点在以为圆心，为半径的圆上，故，代入可得到结果.抛物线的焦点是，准线是，故由抛物线的定义可知，故 ，易知四边形为正方形，故，因此点在以为圆心，为半径的圆上，故，所以 ，即.故答案为：. 　　Mr Robinson was driving to Oxford one cloudy day when he saw a man holding a sign(举着个牌子)above his head．It said CAMBRIDGE on it．Mr Robinson thought it foolish to take the hitchhikers(搭便车的旅行者)because he had read frightening stories of what some of them did to drivers，but he was a kind man，so he stopped and said，“You are on the wrong road for Cambridge．This road goes to Oxford．” 　　“Yes．I know．”answered the man with a smile as he got into Mr Robinson’s car．“That’s where I want to go．I only started to wait here a minute ago，and I knew someone would be likely to(很可能)stop to tell me I was on the wrong road．If I’d held up a sign with OXFORD on it，I might have to wait for an hour for someone to stop．” (1) Mr Robinson didn’t want to take the man because________． [　　] A． he was afraid of the man B． he didn’t know him C． he had heard what some hitchhikers did to drivers D． the man was a foolish man (2) He stopped and told the man________． [　　] A． he would not go to Cambridge B． where he was going C． the right road to Oxford D． the road was for Oxford and not for Cambridge (3) In fact，the man holding a sign was going to________． [　　] A． London B． Oxford C． a town D． Cambridge (4) If the man held up a sign with Oxford on it________． [　　] A． someone would tell him he was right B． someone would take him away C． the cars for Oxford wouldn’t stop D． he would perhaps wait longer (5) The man holding a sign was________． [　　] A． foolish B． clever C． lost his way D． a poor man