\[ y''(x)-y(x) \left (2 \tan ^2(x)+1\right )=0 \] ✓ Mathematica : cpu = 0.131025 (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 = 0.448 (sec), leaf count = 30
\[ \left \{ y \left ( x \right ) ={\frac {i\cos \left ( x \right ) \sin \left ( x \right ) {\it \_C2}+\ln \left ( \cos \left ( x \right ) +i\sin \left ( x \right ) \right ) {\it \_C2}+{\it \_C1}}{\cos \left ( x \right ) }} \right \} \]