\[ y(x) (a \cos (2 x)+b)+y''(x)=0 \] ✓ Mathematica : cpu = 0.0186718 (sec), leaf count = 28
DSolve[(b + a*Cos[2*x])*y[x] + Derivative[2][y][x] == 0,y[x],x]
\[\left \{\left \{y(x)\to c_1 \text {MathieuC}\left [b,-\frac {a}{2},x\right ]+c_2 \text {MathieuS}\left [b,-\frac {a}{2},x\right ]\right \}\right \}\] ✓ Maple : cpu = 0.2 (sec), leaf count = 21
dsolve(diff(diff(y(x),x),x)+(a*cos(2*x)+b)*y(x)=0,y(x))
\[y \left (x \right ) = c_{1} \MathieuC \left (b , -\frac {a}{2}, x\right )+c_{2} \MathieuS \left (b , -\frac {a}{2}, x\right )\]