Đáp án:

 \(40\left( {kV/m} \right)\)

Giải thích các bước giải:

Ta có: 

\(\begin{array}{l}
{r_1} = \dfrac{{a\sqrt 3 }}{2} = \dfrac{{6\sqrt 3 }}{2} = 3\sqrt 3 cm = \dfrac{{3\sqrt 3 }}{{100}}m\\
{r_2} = {r_3} = \dfrac{a}{2} = \dfrac{6}{2} = 3cm = 0,03m\\
{E_1} = \dfrac{{k\left| {{q_1}} \right|}}{{r_1^2}} = \dfrac{{{{9.10}^9}{{.12.10}^{ - 9}}}}{{{{\left( {\dfrac{{3\sqrt 3 }}{{100}}} \right)}^2}}} = 40000\left( {V/m} \right)\\
{E_2} = {E_3} = \dfrac{{k\left| {{q_2}} \right|}}{{r_2^2}} = \dfrac{{{{9.10}^9}{{.12.10}^{ - 9}}}}{{0,{{03}^2}}} = 120000\left( {V/m} \right)\\
{E_{23}} = {E_2} - {E_3} = 0\left( {V/m} \right)\\
 \Rightarrow E = {E_1} = 40000\left( {V/m} \right)
\end{array}\)