theorem
  for x holds fD(sin,h).x = 2*(cos((2*x+h)/2)*sin(h/2))
proof
  let x;
  fD(sin,h).x = sin(x+h)-sin(x) by DIFF_1:3
    .= 2*(cos((x+(h+x))/2)*sin((x+h-x)/2)) by SIN_COS4:16;
  hence thesis;
end;
