\[ y(x) (a \cos (2 x)+b)+y''(x)=0 \] ✓ Mathematica : cpu = 0.019041 (sec), leaf count = 28
\[\left \{\left \{y(x)\to c_1 \text {MathieuC}\left [b,-\frac {a}{2},x\right ]+c_2 \text {MathieuS}\left [b,-\frac {a}{2},x\right ]\right \}\right \}\] ✓ Maple : cpu = 2.273 (sec), leaf count = 21
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,{\it MathieuC} \left ( b,-{\frac {a}{2}},x \right ) +{\it \_C2}\,{\it MathieuS} \left ( b,-{\frac {a}{2}},x \right ) \right \} \]