Khi tăng đồng t

* Hướng dẫn giải

\(\begin{array}{l} \left\{ \begin{array}{l} F = k\frac{{\left| {{q_1}{q_2}} \right|}}{{{r^2}}}\\ F' = k\frac{{\left| {3{q_1}{q_2}} \right|}}{{{{(3r)}^2}}} = k\frac{{\left| {{q_1}{q_2}} \right|}}{{{r^2}}} \end{array} \right.\\ \Rightarrow F' = F \end{array}\)