(1) 串联电流相同
U1+U2=U
电阻两端电压U1=IR； P=UI,灯泡两端电压U2=P/I
IR+P/I=U
10I+0.4/I=4
10I²-4I+0.4=0
25I²-10I+1=0
(5I-1)²=0
I=0.2A
R的电压U1=IR=2V
(2)
P=UI,U=IR,P=I²R
灯电阻 R2=P/I²=0.4W/(0.2A)²=10Ω
灯额定电压为3.8V,额定电流就为 0.38A
P额=3.8V×0.38A=1.444W