a/ Ur1=CănP.R=4,5V
Ir1=0,6=Id2+ Ir2đ1
Ud2 =U r2d1=4,5V
Rd2=12V => Id2=0,375(A)
=> Ir2d1=0,6-0,375=0,225=Id1=I r2( nối tiếp)
Ur2d1=Ur2 + Ud1=0,225.R2+ 0,225.9=4,5=> R=11(ôm)
b/
để 2 đèn sáng bt => Ud2=6V ,Ud1=3V => Ur2=3V Do Ir2=Id1 Ur2=Ud1 => R2=Rd1=9(ôm)
UCB=6V => UAC=Ur1=3V
Icb= Id2 + Ir1d2=1/3 + 3/6=5/6(A) =>R1=3,6(ôm)