\[ y''(x)+\tan (x) y'(x)+y(x) \cos ^2(x)=0 \] ✓ Mathematica : cpu = 0.0347814 (sec), leaf count = 18
\[\left \{\left \{y(x)\to c_2 \sin (\sin (x))+c_1 \cos (\sin (x))\right \}\right \}\] ✓ Maple : cpu = 0.108 (sec), leaf count = 15
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,\sin \left ( \sin \left ( x \right ) \right ) +{\it \_C2}\,\cos \left ( \sin \left ( x \right ) \right ) \right \} \]