\[ \boxed { 2\,f \left ( x \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +2\,f \left ( x \right ) \left ( y \left ( x \right ) \right ) ^{2}- \left ( {\frac {\rm d}{{\rm d}x}}f \left ( x \right ) \right ) y \left ( x \right ) -2\, \left ( f \left ( x \right ) \right ) ^{2}=0} \]
Mathematica: cpu = 0.077510 (sec), leaf count = 38 \[ \left \{\left \{y(x)\to i \sqrt {f(x)} \tan \left (c_1+i \int _1^x -\sqrt {f(K[1])} \, dK[1]\right )\right \}\right \} \]
Maple: cpu = 0.031 (sec), leaf count = 23 \[ \left \{ y \left ( x \right ) =i\tan \left ( -i\int \!\sqrt {f \left ( x \right ) }\,{\rm d}x+{\it \_C1} \right ) \sqrt {f \left ( x \right ) } \right \} \]