reserve h,r,r1,r2,x0,x1,x2,x3,x4,x5,x,a,b,c,k for Real,
  f,f1,f2 for Function of REAL,REAL;

theorem
  (for x holds f.x = k/x & x<>0 & x+h<>0) implies for x holds fD(f,h).x
  = (-k*h)/((x+h)*x);
