如图所示,电源电压保持不变, R1R2为阻值未知的定值电阻。在闭合开关S的情况下,断开S1时,电压表的读数U1=4V,电流表读数I1=0.1A;闭合S1时,电压表的读数U1=6V。求: (1)电源电压U; (2)定值电阻R1R2的阻值; (3)闭合开关S的情况下, S1断开与闭合时电路中的电功率之比。 答案:解:(1)闭合开关S、S1,,R2被短路, 电源电压U=U1=6V (2)当开关S闭合,S1断开,R1 R2串联 (3)由得 P1 :P2=:=:===2:3 ( 2分) When I was only 9 years old, my mother _______ me travel to Nanjing alone. A. encouraged B. made C. agreed D. invited