\[ \left (x^2+1\right ) y'(x)-x \left (x^2+1\right ) \cos ^2(y(x))+x \sin (y(x)) \cos (y(x))=0 \] ✓ Mathematica : cpu = 0.353822 (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 = 0.64 (sec), leaf count = 159
\[ \left \{ y \left ( x \right ) ={\frac {1}{2}\arctan \left ( 6\,{\frac {\sqrt {{x}^{2}+1} \left ( \sqrt {{x}^{2}+1}{x}^{2}+\sqrt {{x}^{2}+1}+3\,{\it \_C1} \right ) }{ \left ( 6\,{\it \_C1}\,{x}^{2}+6\,{\it \_C1} \right ) \sqrt {{x}^{2}+1}+{x}^{6}+3\,{x}^{4}+12\,{x}^{2}+9\,{{\it \_C1}}^{2}+10}},{ \left ( \left ( -6\,{\it \_C1}\,{x}^{2}-6\,{\it \_C1} \right ) \sqrt {{x}^{2}+1}-{x}^{6}-3\,{x}^{4}+6\,{x}^{2}-9\,{{\it \_C1}}^{2}+8 \right ) \left ( \left ( 6\,{\it \_C1}\,{x}^{2}+6\,{\it \_C1} \right ) \sqrt {{x}^{2}+1}+{x}^{6}+3\,{x}^{4}+12\,{x}^{2}+9\,{{\it \_C1}}^{2}+10 \right ) ^{-1}} \right ) } \right \} \]