theorem
  (for x holds f.x = sqrt x) & x+h/2>0 & x-h/2>0 implies
  cD(f,h).x = sqrt (x+h/2) - sqrt (x-h/2)
proof
  assume
A1:for x holds f.x = sqrt x;
  cD(f,h).x = f.(x+h/2) - f.(x-h/2) by DIFF_1:5
    .= sqrt (x+h/2) - f.(x-h/2) by A1
    .= sqrt (x+h/2) - sqrt (x-h/2) by A1;
  hence thesis;
end;
