\left\{\text{x1}'(t)=a \text{x2}(t)+b \text{x3}(t) \cos (c t)+b \text{x4}(t) \sin (c t),\text{x2}'(t)=-a \text{x1}(t)+b \text{x3}(t) \sin (c t)-b \text{x4}(t) \cos (c t),\text{x3}'(t)=a \text{x4}(t)-b \text{x1}(t) \cos (c t)-b \text{x2}(t) \sin (c t),\text{x4}'(t)=-a \text{x3}(t)-b \text{x1}(t) \sin (c t)+b \text{x2}(t) \cos (c t)\right\}