Đáp án:

 \(33V\)

Giải thích các bước giải:

Ta có:

\(\begin{array}{l}
{U_{{V_2}}} = {U_{{V_3}}} + {U_{{R_1}}} = {U_{{V_3}}} + \dfrac{{{U_{{V_3}}}}}{{{R_V}}}.R = 12 + 12.\dfrac{R}{{{R_V}}}\\
 \Rightarrow 18 = 12 + 12.\dfrac{R}{{{R_V}}} \Rightarrow \dfrac{R}{{{R_V}}} = \dfrac{1}{2}\\
{I_{{R_2}}} = \dfrac{{{U_{{V_2}}}}}{{{R_V}}} + \dfrac{{{U_{{V_3}}}}}{{{R_V}}} = \dfrac{{18}}{{{R_V}}} + \dfrac{{12}}{{{R_V}}} = \dfrac{{30}}{{{R_V}}}\\
{U_{{V_1}}} = {U_{{V_2}}} + {U_{{R_2}}} = {U_{{V_2}}} + {I_{{R_2}}}.R\\
 \Rightarrow {U_{{V_1}}} = 18 + 30.\dfrac{R}{{{R_V}}} = 18 + 15 = 33V
\end{array}\)