\[\begin{array}{l}
{\left( {\frac{x}{A}} \right)^2} + {\left( {\frac{v}{{{v_{\max }}}}} \right)^2} = 1\\
v = \frac{{{v_{\max }}}}{2} \to x = \frac{{A\sqrt 3 }}{2}\\
{W_t} = \frac{1}{2}k{x^2} = \frac{1}{2}k.\frac{3}{4}{A^2} = \frac{3}{4}W\\
\to \frac{{{W_d}}}{W} = \frac{{W - {W_t}}}{W} = \frac{1}{4}
\end{array}\]