题目

A teacher stood in front of his history class of twenty students just before handing out the final exam. His students sat quietly and waited for him to speak. “It’s been a pleasure teaching you this term,” he said. “You’ve all worked very hard, so I have a pleasant surprise for you. Everyone who chooses not to take the final exam will get a ‘B’ for the course.” Most of the students jumped out of their seats. They thanked the teacher happily, and walked out of the classroom. Only a few students stayed. The teacher looked at them. “This is your last chance,” he said. 如图，在△中， ，，垂足为.若，，求△的周长（结果保留根号）.