\[ -a \left (\tan ^2(y(x))+1\right )+y'(x)+\tan (x) \tan (y(x))=0 \] ✗ Mathematica : cpu = 50.5929 (sec), leaf count = 0 , could not solve
DSolve[Tan[x]*Tan[y[x]] - a*(1 + Tan[y[x]]^2) + Derivative[1][y][x] == 0, y[x], x]
✗ Maple : cpu = 0. (sec), leaf count = 0 , could not solve
dsolve(diff(y(x),x)-a*(1+tan(y(x))^2)+tan(y(x))*tan(x) = 0,y(x))