\[
\boxed{
 \left\{ {\frac {\rm d}{{\rm d}t}}{\it x1} \left( t \right) =a{\it x2} \left( t \right) +b{\it x3} \left( t \right) \cos \left( ct \right) +b{\it x4} \left( t \right) \sin \left( ct \right) ,{\frac {\rm d}{{\rm d}t}}{\it x2} \left( t \right) =-a{\it x1} \left( t \right) +b{\it x3} \left( t \right) \sin \left( ct \right) -b{\it x4} \left( t \right) \cos \left( ct \right) ,{\frac {\rm d}{{\rm d}t}}{\it x3} \left( t \right) =-b{\it x1} \left( t \right) \cos \left( ct \right) -b{\it x2} \left( t \right) \sin \left( ct \right) +a{\it x4} \left( t \right) ,{\frac {\rm d}{{\rm d}t}}{\it x4} \left( t \right) =-b{\it x1} \left( t \right) \sin \left( ct \right) +b{\it x2} \left( t \right) \cos \left( ct \right) -a{\it x3} \left( t \right)  \right\} }
\]