\[
\boxed{
 \left( {\frac {\rm d}{{\rm d}x}}y \left( x \right)  \right) ^{2}+2\,f \left( x \right) y \left( x \right) {\frac {\rm d}{{\rm d}x}}y \left( x \right) +g \left( x \right)  \left( y \left( x \right)  \right) ^{2}- \left( g \left( x \right) - \left( f \left( x \right)  \right) ^{2} \right) {{\rm e}^{-2\,\int_{a}^{x}\!f \left( {\it xp} \right) \,{\rm d}{\it xp}}}=0}
\]