theorem
  for x holds fD(cos,h).x = -2*(sin((2*x+h)/2)*sin(h/2))
proof
  let x;
  fD(cos,h).x = cos(x+h)-cos(x) by DIFF_1:3
    .= -2*(sin((x+(x+h))/2)*sin((x+h-x)/2)) by SIN_COS4:18;
  hence thesis;
end;
