\[ \left (x^2+1\right ) y'(x)-x \left (x^2+1\right ) \cos ^2(y(x))+x \sin (y(x)) \cos (y(x))=0 \] ✓ Mathematica : cpu = 0.243022 (sec), leaf count = 40
\[\left \{\left \{y(x)\to \tan ^{-1}\left (\frac {-6 c_1 \sqrt {x^2+1}+x^4+2 x^2+1}{3 \left (x^2+1\right )}\right )\right \}\right \}\] ✓ Maple : cpu = 0.96 (sec), leaf count = 25
\[ \left \{ y \left ( x \right ) =\arctan \left ( {\frac {1}{3} \left ( \left ( {x}^{2}+1 \right ) ^{{\frac {3}{2}}}+3\,{\it \_C1} \right ) {\frac {1}{\sqrt {{x}^{2}+1}}}} \right ) \right \} \]