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