\left\{x''(t)=c^2 x(t) \left(3 \cos ^2(a t+b)-1\right)+\frac{3}{2} c^2 y(t) \sin (2 a b t),y''(t)=\frac{3}{2} c^2 x(t) \sin (2 a b t)+c^2 y(t) \left(3 \sin ^2(a t+b)-1\right)\right\}