题目

                                           HOUSE TO RENT         Older house to rent in the city center. One bedroom and one kitchen. Near important stations. Buses pass back door.          $ 475/mo.   Telephone: 3320178        (ENGLISH NEWS PAPER, March 19) English News Paper needs a foreign editor. He / She must have worked in China for over two years. British nationality is necessary. The salary(薪水) is $ 100,000 a year. The term is three years. In ten days this piece of ad will be available(有效的).          Tel 半径为r的圆的面积S(r)＝r2,周长C(r)=2r，若将r看作(0，＋∞)上的变量，则(r2)’＝2r ；对于半径为R的球，若将R看作(0，＋∞)上的变量，请你写出类似于上述的式子:_______________________.