iL(0+)=iL(0-)=8/(12+6)=4/9A
u3（0+）=0V
iL(∞)=[8/(12+6//3)]*3/(6+3)=4/21 A
u3(∞)=(∞)=[8/(12+6//3)]*[6/(6+3)]*3=8/7V
R0=R2+R1//R3=42/5 Ω
时间常数=L/R0=1/84S
iL(t)=iL(∞)+[iL(0+)-iL(∞)]exp(-84t)=4/21+(16/63 )exp(-84t)
u3（t）=u3（∞）+[u3（0+）-U3(∞)]exp(-84t)=8/7[1-exp(-84t)]