\[
\boxed{
 \left\{  \left( {t}^{2}+1 \right) {\frac {\rm d}{{\rm d}t}}x \left( t \right) =-tx \left( t \right) +y \left( t \right) , \left( {t}^{2}+1 \right) {\frac {\rm d}{{\rm d}t}}y \left( t \right) =-x \left( t \right) -ty \left( t \right)  \right\} }
\]