\[
\boxed{
f \left(  \left( y \left( x \right)  \right) ^{2}+{x}^{2} \right) \sqrt { \left( {\frac {\rm d}{{\rm d}x}}y \left( x \right)  \right) ^{2}+1}-x{\frac {\rm d}{{\rm d}x}}y \left( x \right) +y \left( x \right) =0}
\]