\[ y(x) \left (a+\frac {f'(x)}{2}+\frac {f(x)^2}{4}\right )+f(x) y'(x)+y''(x)=0 \] ✓ Mathematica : cpu = 0.0304972 (sec), leaf count = 76
\[\left \{\left \{y(x)\to c_1 \exp \left (-\frac {1}{2} \int _1^xf(K[1])dK[1]-i \sqrt {a} x\right )-\frac {i c_2 \exp \left (-\frac {1}{2} \int _1^xf(K[1])dK[1]+i \sqrt {a} x\right )}{2 \sqrt {a}}\right \}\right \}\] ✓ Maple : cpu = 0.035 (sec), leaf count = 33
\[y \relax (x ) = {\mathrm e}^{-\frac {\left (\int f \relax (x )d x \right )}{2}} \left (\sinh \left (\sqrt {-a}\, x \right ) c_{1}+\cosh \left (\sqrt {-a}\, x \right ) c_{2}\right )\]