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