\[ a \cos \left (y'(x)\right )+b y'(x)+x=0 \] ✗ Mathematica : cpu = 0.0122231 (sec), leaf count = 0 , could not solve
DSolve[x + a*Cos[Derivative[1][y][x]] + b*Derivative[1][y][x] == 0, y[x], x]
✓ Maple : cpu = 0.04 (sec), leaf count = 18
\[ \left \{ y \left ( x \right ) =\int \!{\it RootOf} \left ( a\cos \left ( {\it \_Z} \right ) +b{\it \_Z}+x \right ) \,{\rm d}x+{\it \_C1} \right \} \]