\[ \left (x^2+1\right ) y'(x)-x \left (x^2+1\right ) \cos ^2(y(x))+x \sin (y(x)) \cos (y(x))=0 \] ✓ Mathematica : cpu = 0.416217 (sec), leaf count = 40
\[\left \{\left \{y(x)\to \tan ^{-1}\left (\frac {x^4+2 x^2-6 c_1 \sqrt {x^2+1}+1}{3 \left (x^2+1\right )}\right )\right \}\right \}\] ✓ Maple : cpu = 0.733 (sec), leaf count = 159
\[\left \{y \left (x \right ) = \frac {\arctan \left (\frac {6 \sqrt {x^{2}+1}\, \left (\sqrt {x^{2}+1}\, x^{2}+3 c_{1}+\sqrt {x^{2}+1}\right )}{x^{6}+3 x^{4}+9 c_{1}^{2}+12 x^{2}+\left (6 c_{1} x^{2}+6 c_{1}\right ) \sqrt {x^{2}+1}+10}, \frac {-x^{6}-3 x^{4}-9 c_{1}^{2}+6 x^{2}+\left (-6 c_{1} x^{2}-6 c_{1}\right ) \sqrt {x^{2}+1}+8}{x^{6}+3 x^{4}+9 c_{1}^{2}+12 x^{2}+\left (6 c_{1} x^{2}+6 c_{1}\right ) \sqrt {x^{2}+1}+10}\right )}{2}\right \}\]