\[ y''(x)+y(x) \left (-2 \tan ^2(x)-1\right )=0 \] ✓ Mathematica : cpu = 0.158162 (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 = 0.167 (sec), leaf count = 30
\[\left \{y \left (x \right ) = \frac {i c_{2} \cos \left (x \right ) \sin \left (x \right )+c_{2} \ln \left (\cos \left (x \right )+i \sin \left (x \right )\right )+c_{1}}{\cos \left (x \right )}\right \}\]