解：?$(1)R_1=\frac {U_1}{I}=\frac {4\ \mathrm {V}}{0.4\ \mathrm {A}}=10 \ \mathrm {Ω}$?
?$ U_{2}=U-U_{1}=6\ \mathrm {V}-4\ \mathrm {V}=2\ \mathrm {V}$?
?$ R_{2}=\frac {U_{2}}{I}=\frac {2\ \mathrm {V}}{0.4\ \mathrm {A}}=5 \ \mathrm {Ω}$?
?$ (2)I_{1}=\frac {U}{R_{1}}=\frac {6\ \mathrm {V}}{10 \ \mathrm {Ω}}=0.6\ \mathrm {A}$?
?$ I_2=\frac {U}{R_2}=\frac {6\ \mathrm {V}}{5 \ \mathrm {Ω}}=1.2\ \mathrm {A}$?
?$ I'=I_1+I_2=0.6\ \mathrm {A}+1.2\ \mathrm {A}=1.8\ \mathrm {A}$?
