如图所示的电路中,已知四个电阻的阻值为R1=1欧,R2=2欧,R3=3欧,R4=4欧,电源电动势E=4V,内阻r=0.2欧,试求:(1)S闭合,S'断开时,通过R1和R2的电流之比I1/I2。(2)S和S'都闭合是的总电流I'