\[ y'(x)=\frac {\frac {1}{2} x^3 \cos (2 y(x))+\frac {x^3}{2}-\frac {1}{2} \sin (2 y(x))}{x} \] ✓ Mathematica : cpu = 0.286483 (sec), leaf count = 21
\[\left \{\left \{y(x)\to \tan ^{-1}\left (\frac {x^4+2 c_1}{4 x}\right )\right \}\right \}\] ✓ Maple : cpu = 0.831 (sec), leaf count = 17
\[y \relax (x ) = \arctan \left (\frac {x^{4}+8 c_{1}}{4 x}\right )\]