$ 解：(1)I_{ }=\frac {U_{ 2}}{R_{ 2}}=\frac {{ 8 }\ \text {V}}{{ 80 }Ω}={ 0.1 }\ \text {A}$
$ (2)U_1=U-U_2=9\ \text {V}-8\ \text {V}=1\ \text {V}$
$ R_{ }=\frac {U_{ 1 }}{I_{ }}=\frac {{ 1 }\ \text {V}}{{ 0.1 }\ \text {A}}={ 10 }Ω$
$ (3)U_{ 1 }'=I_{ }'R_{ 1}={ 0.3 }\ \text {A}×{ 10 }\ Ω={ 3 }\ \text {V}$
$ U_2=U-U_1=9\ \text {V}-3\ \text {V}=6\ \text {V}$
