\[
\boxed{
{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) =-{\frac {x{\frac {\rm d}{{\rm d}x}}y \left( x \right) }{f \left( x \right) }}+{\frac {y \left( x \right) }{f \left( x \right) }}=0}
\]