\[ \boxed { \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+ \left ( y \left ( x \right ) \right ) ^{2}- \left ( f \left ( x \right ) \right ) ^{2}=0} \]
Mathematica: cpu = 22.438849 (sec), leaf count = 23 \[ \text {DSolve}\left [-f(x)^2+y'(x)^2+y(x)^2=0,y(x),x\right ] \]
Maple: cpu = 1.404 (sec), leaf count = 0 \[ \text {could not solve} \]