R1R2并联,因此S闭合时,R2上的电流为0.6 - 0.2 = 0.4A
因此电阻R2 = P/I² = 1.2 ÷ 0.4² = 7.5Ω
电源电压为7.5×0.4 = 30V,因此R1 = 30÷0.2 = 15Ω