\[ -a x-b \sin (x)-c \cos (x)+y^{(n)}(x)+2 y^{(3)}(x)+y'(x)=0 \] ✓ Mathematica : cpu = 0.645438 (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 = 0.304 (sec), leaf count = 78
\[ \left \{ y \left ( x \right ) =-{\frac {\cos \left ( x \right ) cx}{2}}-{\frac {\sin \left ( x \right ) c{x}^{2}}{8}}+{\frac {21\,c\sin \left ( x \right ) }{32}}-{\frac {\sin \left ( x \right ) bx}{2}}-{\frac {3\,b\cos \left ( x \right ) }{4}}+{\frac {\cos \left ( x \right ) b{x}^{2}}{8}}+{\frac {a{x}^{2}}{2}}+{\it \_C1}\,\sin \left ( x \right ) -{\it \_C2}\,\cos \left ( x \right ) +\sin \left ( x \right ) {\it \_C3}\,x+\cos \left ( x \right ) {\it \_C3}-\cos \left ( x \right ) {\it \_C4}\,x+\sin \left ( x \right ) {\it \_C4}+{\it \_C5} \right \} \]