reserve n,m for Element of NAT;
reserve h,k,r,r1,r2,x,x0,x1,x2,x3 for Real;
reserve f,f1,f2 for Function of REAL,REAL;

theorem
  [!f,x,x+h!] = (fD(f,h).x)/h
proof
  [!f,x,x+h!] = (-(f.(x+h)-f.x))/(-h) .= (f.(x+h)-f.x)/h by XCMPLX_1:191
    .= (fD(f,h).x)/h by DIFF_1:3;
  hence thesis;
end;
