\[ y(x) \left (a \cos ^2(x)+b\right )+y''(x)=0 \] ✓ Mathematica : cpu = 0.0469676 (sec), leaf count = 40
\[\left \{\left \{y(x)\to c_1 \text {MathieuC}\left [\frac {a}{2}+b,-\frac {a}{4},x\right ]+c_2 \text {MathieuS}\left [\frac {a}{2}+b,-\frac {a}{4},x\right ]\right \}\right \}\]
✓ Maple : cpu = 3.908 (sec), leaf count = 39
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,{\it MathieuC} \left ( -{\frac {a}{2}}-b,{\frac {a}{4}},ix \right ) +{\it \_C2}\,{\it MathieuS} \left ( -{\frac {a}{2}}-b,{\frac {a}{4}},ix \right ) \right \} \]