题目

All over the world the need for more laptops and computers increases each year. Networks have been part of our life. Probably everyone with a portable device (移动设备) has once logged on to a public WiFi network without paying for it, while they are having a coffee, on a train, in a shopping mall or at a hotel. However, is this kind of public WiFi network safe?Some networks are better protected than others because encryption (加密) methods are used. Therefore, they are safer than others. But a survey in 2015 showed that more than 950 million records were not protected, including address如图，在△ABC中，∠B=45°，∠C=30°，AD⊥BC于D，BD=6，求DC的长和△ABC的面积（结果保留根号）．