题目

Good afternoon, and welcome to England. We hope that your visit here will be a pleasant one. Today, I would like to draw your attention to a few of our rules. The first one is about drinking. Now, you may not buy alcohol in this country if you are under 18 years of age, nor may your friends buy it for you. Secondly, noise. Enjoy yourselves by all means, but please don’t make unnecessary noise, particularly at night. We ask you to respect other people who may wish to be quiet. Thirdly, crossing the road. Be careful. The traffic moves on  the left side of the road in this country. Us 在Rt△ABC中，∠C＝90°，a＝5，b＝12，c＝________．