题目

Most of the people use smartphones. They like using instant messaging apps (即时通讯应用程序) to communicate with friends. These apps have become a big hit because they are convenient and more importantly, free. Let’s have a look at three of the most popular instant messaging apps in the world. WeChat: It is a mobile messaging app by China’s Internet company Tencent in 2011. Now it has become more popular than Weibo, which appeared in 2010. It is more than an instant messaging app. Users can pay bills, play games, post photos and so on.WhatsApp: WhatsApp Messenger is a mobile messa已知直线l1：x+2ay-1=0，l2：（3a+1）x-ay+1=0．（1）当l1∥l2时，求a的值；（2）当l1⊥l2时，求a的值．