US1=I1R1-I2R2+US2
US2=I2R2+I3R3
US1=I1R1+I3R3
解之。
用戴维宁定理，等效电压U0=(U1/R1+U2/R2)X(R1XR2/(R1+R2))=100V等效电阻R0=R1XR2/(R1+R20=4欧所以I3=U0/(R0+R3)
