①∀a，b∈N^*，a≠b有af（a）+bf（b）＞af（b）+bf（a）；

②∀n∈N^*，有f（f（n））=3n，

则f（1）+f（6）+f（28）=____．

一只灯泡与变阻器串联后接在54伏的电源上，灯泡的额定电压是36伏，电阻是15欧，为使灯泡正常发光，此变阻器的规格应为（　　）

综合性学习

    I have a small room. There is a bed, a desk and two chairs in it. There are some books and a pencil-box on the desk. There is a photo(照片)on the wall. It's a photo of my family. There is a map of China and two pictures on the wall, too. My room is nice. I like it very much.