在公路上，常常可以看到汽车的后面挂着一个牌子“保持车距”．你能解释一下这是做什么用的吗?

若x，y为实数，且，则=____ 。

根据短文理解，选择正确答案填空。 

    Jim is a junior high school student. He gets up 16:30 every morning. His home is near his school. He 2home after breakfast. He goes to school on foot. This is good for his 3. He has four lessons in the morning. He has lunch at home. After lunch he goes to school again. In the afternoon, Jim has lessons . He likes English and PE lessons. He has PE lessons on Tuesday and Friday. He is good at4football and basketball. He is also good at the high jump. His PE teacher likes5 very much. He often plays football with his classmates after school.