题目

As we all know, animals are our good friends. Many kids love animals very much. They like to go to the zoo every Saturday, Sunday or even every day on weekdays. But do you want to go to the zoo at night?    Usually, the zoos aren’t open at night. But if you are in Singapore, you may have a chance to visit a zoo at night. There is a night zoo in Singapore. Its name is Night Safari. It is open at night. Why? That’s because many animals only wake up （醒来）in the evening, like tigers and wolves. During the day, they like to sleep, so it’s the best time to watch them at night. I（本小题满分14分） 如图所示， 平面，底面为菱形， 为的中点． （1）求证：平面;  （2）求证: //平面；   (3) 求二面角的平面角的大小.