\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +a{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +f \left ( x \right ) \sin \left ( y \left ( x \right ) \right ) =0} \]
Mathematica: cpu = 0.055507 (sec), leaf count = 23 \[ \text {DSolve}\left [a y'(x)+f(x) \sin (y(x))+y''(x)=0,y(x),x\right ] \]
Maple: cpu = 0.593 (sec), leaf count = 0 \[ \text {could not solve} \]