例2电路如图所示。已知us1(=24V,s2(=4V, ls3()=6V,R1=192,R2=292和R3=42 求电流i()和电压unt() us +R1a+ u S2 R u R3 b例2 电路如图所示。已知uS1 (t) =24V, uS2 (t)=4V, uS3 (t) =6V, R1 =1, R2 =2和R3 =4。 求电流i (t)和电压uab (t)