\[ \left (y'(x)^2+1\right ) \left (a x+\tan ^{-1}\left (y'(x)\right )\right )+y'(x)=0 \] ✓ Mathematica : cpu = 1.46901 (sec), leaf count = 51
\[\text {Solve}\left [\left \{y(x)=\frac {1}{a \left (\text {K$\$$137866}^2+1\right )}+c_1,x=\frac {\text {K$\$$137866}^2 \left (-\tan ^{-1}(\text {K$\$$137866})\right )-\text {K$\$$137866}-\tan ^{-1}(\text {K$\$$137866})}{a \left (\text {K$\$$137866}^2+1\right )}\right \},\{y(x),\text {K$\$$137866}\}\right ]\] ✓ Maple : cpu = 0.09 (sec), leaf count = 30
\[ \left \{ y \left ( x \right ) =\int \!\tan \left ( {\it RootOf} \left ( ax \left ( \tan \left ( {\it \_Z} \right ) \right ) ^{2}+ \left ( \tan \left ( {\it \_Z} \right ) \right ) ^{2}{\it \_Z}+ax+\tan \left ( {\it \_Z} \right ) +{\it \_Z} \right ) \right ) \,{\rm d}x+{\it \_C1} \right \} \]