\[ -f(x)^2+y'(x)^2+y(x)^2=0 \] ✗ Mathematica : cpu = 18.1209 (sec), leaf count = 0 , could not solve
DSolve[-f[x]^2 + y[x]^2 + Derivative[1][y][x]^2 == 0, y[x], x]
✗ Maple : cpu = 0. (sec), leaf count = 0 , could not solve
dsolve(diff(y(x),x)^2+y(x)^2-f(x)^2 = 0,y(x))