∵R1、R2串联
∴U源=U1+U2,I总=I1=I2
1、R1两端电压:U1=U源-U2=12V-4V=8V
2、I总=I2=I1=U1/R1=8V/8Ω=1A
3、R2=U2/I2=4V/1A=4Ω