题目

 --- Tom is not here these days. Is he ill?    ---  ______. His mother told me that he was in hospital. A. I don't think so      B. I hope not     C. I don't mind        D. I'm afraid so 答案：D为了研究串联电路电阻问题，小文设计了如图20所示的实验电路，其中R1是电阻箱，R2是定值电阻，R1与R2串联的总电阻用R表示．电流表测量的是串联电路中的电流I，电压表测量的是A、B两点间的电压U．实验记录的数据如下表所示．（1）填写下表中的空格处．R1/Ω152025354555U/V666666I/A0.30.240.20.150.120.1R/Ω2025304050（2）请根据实验条件及表中实验数据，归纳出总电阻R跟电阻R1的关系：    ．