\[ \left \{x'(t)=a y(t),y'(t)=-a x(t)\right \} \] ✓ Mathematica : cpu = 0.0091008 (sec), leaf count = 39
\[\{\{x(t)\to c_1 \cos (a t)+c_2 \sin (a t),y(t)\to c_2 \cos (a t)-c_1 \sin (a t)\}\}\] ✓ Maple : cpu = 0.058 (sec), leaf count = 35
\[\{x \relax (t ) = c_{1} \sin \left (a t \right )+c_{2} \cos \left (a t \right ), y \relax (t ) = \cos \left (a t \right ) c_{1}-\sin \left (a t \right ) c_{2}\}\]