theorem
  for x holds bD(cos,h).x = -2*(sin((2*x-h)/2)*sin(h/2))
proof
  let x;
  bD(cos,h).x = cos(x)-cos(x-h) by DIFF_1:4
    .= -2*(sin((x+(x-h))/2)*sin((x-(x-h))/2)) by SIN_COS4:18;
  hence thesis;
end;
