两电阻串联 ,通过R1,R2的电流相等
U1/R1=U2/R2
U1/U2=R1/R2
2/3=4/R2
R2=6
I1/I2=1/1=1
R1/R2=(U1/I1)/(U2/I2)=1/2*4/1=2/1