标有“6V 6W”、“9V 3W”字样的灯L1和L2,串联在某电源上,设灯丝电阻不变,要保证两灯安全工作,电源电压不应超

R1 = U^2/P = 6*6/6 = 6欧
R2 = U^2/P = 9*9/3 = 27欧
串联,R = R1+R2 = 6+27 = 33欧
保证两灯安全工作,电流不能超过I=P2/U2 = 3/9 = 1/3
电源电压不应超过：U = IR = 1/3 * 33 = 11伏
并联,两支路电流比,I1:I2 = R2:R1 = 27:6 = 9:2
要保证安全工作,I1不能大于6/6=1安,I2 = 1 * 2/9 = 2/9安
所以干路电流不超过：1 + 2/9 = 11/9安