\[
\boxed{
{\frac {\rm d}{{\rm d}x}}y \left( x \right) =-{\frac {x}{\sqrt { \left( y \left( x \right)  \right) ^{2}}} \left( -{x}^{-1}-{\it \_F1} \left(  \left( y \left( x \right)  \right) ^{2}-2\,x \right)  \right) }=0}
\]