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