reserve h,r,r1,r2,x0,x1,x2,x3,x4,x5,x,a,b,c,k for Real,
  f,f1,f2 for Function of REAL,REAL;

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;
