Đáp án:
$\begin{array}{l}
\frac{a}{3} = \frac{3}{b} = \frac{b}{a}\\
\Rightarrow \left\{ \begin{array}{l}
a.b = {3^2}\\
{b^2} = 3a \Rightarrow a = \frac{{{b^2}}}{3}\\
{a^2} = 3b
\end{array} \right.\\
\Rightarrow \frac{{{b^2}}}{3}.b = {3^2} \Rightarrow {b^3} = {3^3} \Rightarrow b = 3 \Rightarrow a = 3\\
= a = b = 3
\end{array}$
