R2+R4=4(Ω)(R2+R4)与R3并联电阻=4×2/(4+2)=4/3(Ω)R总=4/3+2=10/3(Ω)----等效电阻I=Us1/(10/3)=6=1.8(A)U3=6-1.8×2=2.4(v)I3=U3/R3=2.4/2=1.2(A)
总电阻为2欧姆，总电压为6V根据并联电压不变，公式可得I3电流为3a
