\[ y''(x)-y(x) \left (2 \tan ^2(x)+1\right )=0 \] ✓ Mathematica : cpu = 0.18053 (sec), leaf count = 84
\[\left \{\left \{y(x)\to \frac {c_1 \sqrt [4]{1-\cos ^2(x)} \sec (x)}{\sqrt [4]{\cos ^2(x)-1}}-\frac {c_2 \sqrt [4]{1-\cos ^2(x)} \sec (x) \left (\cos (x) \sqrt {1-\cos ^2(x)}-\sin ^{-1}(\cos (x))\right )}{2 \sqrt [4]{\cos ^2(x)-1}}\right \}\right \}\]
✓ Maple : cpu = 0.138 (sec), leaf count = 34
\[ \left \{ y \left ( x \right ) ={\frac {{\it \_C1}}{\cos \left ( x \right ) }}+{\frac {{\it \_C2}\, \left ( i\cos \left ( x \right ) \sin \left ( x \right ) +\ln \left ( \cos \left ( x \right ) +i\sin \left ( x \right ) \right ) \right ) }{\cos \left ( x \right ) }} \right \} \]