\[ y''(x)+y(x)=0 \] ✓ Mathematica : cpu = 0.00569952 (sec), leaf count = 16
\[\left \{\left \{y(x)\to c_2 \sin (x)+c_1 \cos (x)\right \}\right \}\]
✓ Maple : cpu = 0.005 (sec), leaf count = 13
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,\sin \left ( x \right ) +{\it \_C2}\,\cos \left ( x \right ) \right \} \]