Lời giải:

\[{R_{1\,}}nt\,({R_2}\parallel {R_3})\,nt\,{R_4}\]

a) \[{\xi _{bo}} = 3.2 = 6(V)\]

\[{r_{bo}} = 3.0,5 = 1,5(V)\]

Có \[{R_{23}} = \frac{{{R_2}{R_3}}}{{{R_2} + {R_3}}} = 1(\Omega )\]

\[{P_1} = I_1^2.{R_1} = {1,2^2}.1,5 = 2,16\,({\rm{W}})\]

b) \[{P_{\;nguon}} = \xi .I = 6.1,2 = 7,2\,\,({\rm{W}})\]

c) \[\frac{{{A_{c\'o {\rm{ }}\'i ch}}}}{{{A_{tp}}}} = \frac{{\xi .I.t}} = \frac{U}{\xi } = \frac{{I.{R_{td}}}}{{I({R_{td}} + r)}} = \frac{{{R_{td}}}}{{{R_{td}} + r}}\]

\[ \Rightarrow H = \frac{{{A_{c\'o {\rm{ }}\'i ch}}}}{{{A_{tp}}}}.100\% = \frac{{{R_{td}}}}{{{R_{td}} + r}}.100\% = 70\% \]