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 Th22:
  [!f,x-h/2,x+h/2!] = (cD(f,h).x)/h
proof
  [!f,x-h/2,x+h/2!] = [!f,x+h/2,x-h/2!] by DIFF_1:29
    .= (cD(f,h).x)/h by DIFF_1:5;
  hence thesis;
end;
