theorem
  for x holds cD(sin,h).x = 2*(cos(x)*sin(h/2))
proof
  let x;
  cD(sin,h).x = sin(x+h/2)-sin(x-h/2) by DIFF_1:5
    .= 2*(cos((x+h/2+(x-h/2))/2) *sin((x+h/2-(x-h/2))/2)) by SIN_COS4:16;
  hence thesis;
end;
