\[
\boxed{
{\frac {\rm d}{{\rm d}x}}y \left( x \right) =1/2\,{\frac {-\sin \left( 2\,y \left( x \right)  \right) +\cos \left( 2\,y \left( x \right)  \right) {x}^{3}+{x}^{3}}{x}}=0}
\]