题目

We each have a memory（记忆力）. That’s why we can still remember things after a long time. Some people have very good memories and they can easily learn many things by heart, but some people can only remember things when they say or do them again and again. Many of the great men of the world have got surprising memories.       A good memory is a great help in learning a language. Everybody learns his mother language when he is a small child. He hears the sounds, remembers them and then he learns to speak. Some children are living with their parents in foreign countries. They can 如图所示的电路，电源电压保持不变，R1＝20 Ω.当闭合开关S1、S3，断开S2时，电流表的示数为0.3 A；当闭合开关S2、S3，断开S1时，电流表的示数为1.2 A. (1)求电源电压；_________-(2)求电阻R2的阻值；__________(3)当闭合S1，断开S2、S3时，电路消耗的电功率为P1；当闭合S2，断开S1、S3时，电路消耗的电功率为P2，且P1∶P2＝1∶2，求电阻R3的阻值_________--；(4)求整个电路消耗的最小电功率．____________