当a,b两端接入100V电压时c,d两端为20V.当c,d两端接入100V电压时,a,b两端电压为50V,则R1:R2:R3是
当a,b两端接入100V电压,cd电压就是R2两端的电压.
Uab/(2R1+R2)=Ucd/R2
100/(2R1+R2)=20/R2---------------(1)
当c,d两端接入100V电压时,a,b两端电压就是R2两端的电压.
Ucd/(2R3+R2)=Uab/R2
100/(2R3+R2)=50/R2---------------(2)
由(1)得：R1=2*R2
由(2)得：R3=(1/2)*R2
故,R1:R2:R3=2:1:(1/2)=4:2:1