a)Rtđ=\(\dfrac{R1.R2}{R1+R2}=\dfrac{30.20}{30+20}=12\)Ω
I1=\(\dfrac{U1}{R1}=\dfrac{U}{R1}=\dfrac{12}{30}=0,4A\)
I2\(\dfrac{U2}{R2}=\dfrac{U}{R2}=\dfrac{12}{20}=0,6A\)
b) I3=I-I1=1,6-0,4=1,2A
R3=\(\dfrac{U3}{I3}=\dfrac{U}{I3}=\dfrac{12}{1,2}=10\)Ω