\[ f(x) y''(x)+f(x) y(x)+y^{(3)}(x)+y'(x)=0 \] ✗ Mathematica : cpu = 0.0833206 (sec), leaf count = 0 , could not solve
DSolve[f[x]*y[x] + Derivative[1][y][x] + f[x]*Derivative[2][y][x] + Derivative[3][y][x] == 0, y[x], x]
✓ Maple : cpu = 0.223 (sec), leaf count = 36
\[ \left \{ y \left ( x \right ) ={{\rm e}^{ix}} \left ( \int \!{{\rm e}^{-2\,ix}} \left ( \int \!{\it \_C3}\,{{\rm e}^{\int \!i-f \left ( x \right ) \,{\rm d}x}}\,{\rm d}x+{\it \_C2} \right ) \,{\rm d}x+{\it \_C1} \right ) \right \} \]