R1和R2的并联电阻是R1*R2/(R1+R2)=2欧姆R3和R4的串联电阻为R3+R4=4欧姆R3\R4和R5的并联电阻为(R3+R4)*R5/（R3+R4+R5)=2欧姆所以U5=E/(2+2)*2=3VI5=U5/R5=3/4=0.75AU3=U5*R3/(R3+R4)=0.75V
