[tex]\frac{F_{1}}{F_{2}}=\frac{\frac{k.\left | q^{2} \right |}{a^{2}}}{\frac{k.\left | q^{2} \right |}{(a+4.10^{-3})^{2}}}=\frac{k.\left | q^{2} \right |}{a^{2}}.\frac{(a+4.10^{-3})^{2}}{k.\left | q^{2} \right |}=\frac{(a+4.10^{-3})^{2}}{a^{2}}=(\frac{a+4.10^{-3}}{a})^{2}[/tex]
còn [tex] 4.10^{-3}[/tex] (m) =4mm