\[ -\sin (n x)+y''(x)+y(x)=0 \] ✓ Mathematica : cpu = 0.112628 (sec), leaf count = 45
\[\left \{\left \{y(x)\to c_2 \sin (x)+c_1 \cos (x)+\frac {\cos ^2(x) (-\sin (n x))-\sin ^2(x) \sin (n x)}{n^2-1}\right \}\right \}\]
✓ Maple : cpu = 0.078 (sec), leaf count = 26
\[ \left \{ y \left ( x \right ) =\sin \left ( x \right ) {\it \_C2}+\cos \left ( x \right ) {\it \_C1}-{\frac {\sin \left ( nx \right ) }{{n}^{2}-1}} \right \} \]