Ta có: ${F_{hd}} = G\frac{{{m_1}{m_2}}}{{{r^2}}}$
a. ${F_{hd1}} = G\frac{{{m_1}3{m_2}}}{{{r^2}}}$
$\frac{{{F_{hd1}}}}{{{F_{hd}}}} = 3$
b. $\begin{array}{l}
{F_{hd2}} = G\frac{{{m_1}.\frac{1}{2}.{m_2}}}{{{r^2}}}\\
\frac{{{F_{hd2}}}}{{{F_{hd}}}} = \frac{1}{2}
\end{array}$
c. $\begin{array}{l}
{F_{hd3}} = G\frac{{{m_1}.{m_2}}}{{{{(3r)}^2}}}\\
\frac{{{F_{hd3}}}}{{{F_{hd}}}} = \frac{1}{9}
\end{array}$
d. $\begin{array}{l}
{F_{hd4}} = G\frac{{{m_1}.{m_2}}}{{{{(\frac{1}{2}r)}^2}}}\\
\frac{{{F_{hd4}}}}{{{F_{hd}}}} = 4
\end{array}$