[tex]E_{A}=k.\frac{q}{r_{A}^{2}}[/tex] (1)
[tex]E_{B}=k.\frac{q}{r_{B}^{2}}[/tex] (2)
Lấy 1 chia 2 => [tex]\left ( \frac{r_{B}}{r_{A}} \right )^{2}[/tex] = [tex]\frac{16}{9}[/tex]
=> [tex]r_{B}=\frac{4r_{A}}{3}[/tex]
[tex]E_{M}=k.\frac{q}{r_{M}^{2}}[/tex] (3)
[tex]r_{M}= \frac{r_{A}+r_{B}}{2}[/tex] = [tex]\frac{7.r_{A}}{6}[/tex]
Lấy 3 chia 1
[tex]\frac{E_{M}}{E_{A}}= (\frac{r_{A}}{r_{M}})^{2}[/tex]
=> [tex]E_{M}=\frac{36}{49}.1600=[/tex] =1175,51 V/m