\[ y'(x)-\cos (a y(x)+b x)=0 \] ✓ Mathematica : cpu = 0.308356 (sec), leaf count = 124
\[\left \{\left \{y(x)\to \frac {-2 \tan ^{-1}\left (\frac {a \tanh \left (\frac {1}{2} \left (c_1 \sqrt {a^2-b^2}-x \sqrt {a^2-b^2}\right )\right )}{\sqrt {a^2-b^2}}+\frac {b \tanh \left (\frac {1}{2} \left (c_1 \sqrt {a^2-b^2}-x \sqrt {a^2-b^2}\right )\right )}{\sqrt {a^2-b^2}}\right )-b x}{a}\right \}\right \}\]
✓ Maple : cpu = 0.075 (sec), leaf count = 65
\[ \left \{ y \left ( x \right ) =-{\frac {1}{a} \left ( bx+2\,\arctan \left ( {\frac {\tanh \left ( 1/2\,{\it \_C1}\,\sqrt {{a}^{2}-{b}^{2}}-1/2\,x\sqrt {{a}^{2}-{b}^{2}} \right ) \sqrt {{a}^{2}-{b}^{2}}}{a-b}} \right ) \right ) } \right \} \]