\[\begin{array}{l}{R_{45}} = \frac{{{R_4}{R_5}}}{{{R_4} + {R_5}}} = 2\,\Omega ;\,\,{R_{345}} = {R_3} + {R_{45}} = 4\,\Omega \\{R_{2345}} = \frac{{{R_2}.{R_{345}}}}{{{R_2} + {R_{345}}}} = 2\,\Omega ;\,\,{R_{12345}} = {R_1} + {R_{2345}} = 8\,\Omega \end{array}\]

a) Vì \({R_{345}} = {R_2}\) nên ta có \({I_2} = {I_{345}} = \frac{}{2} = 0,5{\rm{\;A}}\).

\({U_} = {I_2}{R_2} = 0,5 \cdot 4 = 2{\rm{\;V}}{\rm{.\;}}\)

b) Ta có: \({U_} = {U_} \Rightarrow \frac{}{} = \frac{}{} = 2 \Rightarrow {I_4} = 2{I_5} = 2{\rm{\;A}}\).

Vì \({R_{345}} = {R_2}\) nên ta suy ra: \({I_{345}} = {I_2} = {I_3} = {I_4} + {I_5} = 3{\rm{\;A}}\).

\({U_} = {I_2}{R_2} = 3.4 = 12{\rm{\;V}}{\rm{.\;}}\)