\[ \tan (x) y'(x)+y''(x)+y(x) \cos ^2(x)=0 \] ✓ Mathematica : cpu = 0.0260606 (sec), leaf count = 18
\[\{\{y(x)\to c_2 \sin (\sin (x))+c_1 \cos (\sin (x))\}\}\] ✓ Maple : cpu = 0.178 (sec), leaf count = 15
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,\sin \left ( \sin \left ( x \right ) \right ) +{\it \_C2}\,\cos \left ( \sin \left ( x \right ) \right ) \right \} \]