Đáp án:
$\begin{array}{l}
9{x^2} + 25{y^2} = 225\\
\Rightarrow \frac{{{x^2}}}{{25}} + \frac{{{y^2}}}{9} = 1\\
\Rightarrow \left\{ \begin{array}{l}
{a^2} = 25 \Rightarrow a = 5 \Rightarrow 2a = 10\\
{b^2} = 9 \Rightarrow b = 3 \Rightarrow 2b = 6
\end{array} \right.\\
\Rightarrow S = a.b = 60
\end{array}$