\[ -a \cos (b x)+y''(x)+y(x)=0 \] ✓ Mathematica : cpu = 0.0974877 (sec), leaf count = 47
\[\left \{\left \{y(x)\to \frac {-a \cos ^2(x) \cos (b x)-a \sin ^2(x) \cos (b x)}{b^2-1}+c_2 \sin (x)+c_1 \cos (x)\right \}\right \}\]
✓ Maple : cpu = 0.049 (sec), leaf count = 27
\[ \left \{ y \left ( x \right ) =\sin \left ( x \right ) {\it \_C2}+\cos \left ( x \right ) {\it \_C1}-{\frac {a\cos \left ( bx \right ) }{{b}^{2}-1}} \right \} \]