A设电源电压是U,当开关S1闭合 S2断开时,R1R2串联 U/(R1+R2)=0.2A当开关S1、S2闭合时,R2被短路,只有R1,电流表的示数为0.8A,U/R1=0.8A两式比值解得R1=1/3R2,即R1:R2=1:3