答：电源接ab，cd接电压表。此时R4与R5实际上是断开的。故原电路为R1、R2、R3串联。且R3两端的电压为4V。这样R1与R2两端的总电压为：U12=U-U3=5V-4V=1V电流：I3=I12=U/（R1+R2）=1V/(10Ω)=0.1A故，R3的大小为：R3=U3/I3=4V/0.1A=40Ω
