串联电路电流：I＝P/U＝3/12＝0.25(A)电阻R1两端电压：U1＝18－12＝6(V)电阻R1的阻值：R1＝U/I＝6/0.25＝24(Ω)电阻R1消耗功率：P＝U×U/R＝6×6/24＝1.5(W)或：P＝I×I×R＝0.25×0.25×24＝1.5(W)或：P＝UI＝6×0....