\[ -a x-b \sin (x)-c \cos (x)+y^{(n)}(x)+2 y^{(3)}(x)+y'(x)=0 \] ✓ Mathematica : cpu = 0.677678 (sec), leaf count = 104
\[\left \{\left \{y(x)\to \frac {a x^2}{2}+\frac {1}{8} b \left (x^2-2\right ) \cos (x)-\frac {3}{8} b x \sin (x)-\frac {5}{16} b \cos (x)-\frac {1}{8} c \left (x^2-2\right ) \sin (x)+c_2 x \sin (x)+\frac {9}{16} c \sin (x)+c_1 \sin (x)+c_4 \sin (x)-\frac {5}{8} c x \cos (x)-c_4 x \cos (x)+c_2 \cos (x)-c_3 \cos (x)+c_5\right \}\right \}\]
✓ Maple : cpu = 0.414 (sec), leaf count = 69
\[ \left \{ y \left ( x \right ) ={\frac { \left ( b{x}^{2}+ \left ( -4\,c-8\,{\it \_C4} \right ) x-6\,b-8\,{\it \_C2}+8\,{\it \_C3} \right ) \cos \left ( x \right ) }{8}}+{\frac { \left ( -c{x}^{2}+ \left ( -4\,b+8\,{\it \_C3} \right ) x+6\,c+8\,{\it \_C1}+8\,{\it \_C4} \right ) \sin \left ( x \right ) }{8}}+{\frac {a{x}^{2}}{2}}+{\it \_C5} \right \} \]