\[ \left \{x'(t)=a y(t),y'(t)=-a x(t)\right \} \] ✓ Mathematica : cpu = 0.0095632 (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 = 0.067 (sec), leaf count = 35
\[\{\{x \left (t \right ) = c_{1} \sin \left (a t \right )+c_{2} \cos \left (a t \right ), y \left (t \right ) = c_{1} \cos \left (a t \right )-c_{2} \sin \left (a t \right )\}\}\]