\[ y''(x)+\tan (x) y'(x)+y(x) \cos ^2(x)=0 \] ✓ Mathematica : cpu = 0.0258846 (sec), leaf count = 18
\[\{\{y(x)\to c_2 \sin (\sin (x))+c_1 \cos (\sin (x))\}\}\] ✓ Maple : cpu = 0.151 (sec), leaf count = 15
\[\{y \left (x \right ) = c_{1} \sin \left (\sin \left (x \right )\right )+c_{2} \cos \left (\sin \left (x \right )\right )\}\]