theorem
  for x holds cD(cos,h).x = -2*(sin(x)*sin(h/2))
proof
  let x;
  cD(cos,h).x = cos(x+h/2)-cos(x-h/2) by DIFF_1:5
    .= -2*(sin((x+h/2+(x-h/2))/2) *sin((x+h/2-(x-h/2))/2)) by SIN_COS4:18;
  hence thesis;
end;
