reserve x,x1,x2,x3 for Real;

theorem
  (cos(x))|^3=(3*cos(x)+cos(3*x))/4
proof
  (3*cos(x)+cos(3*x))/4 =(3*cos(x)+(4*(cos(x))|^3 - 3*cos(x)))/4 by Th17
    .=(4*(cos(x))|^3)/4;
  hence thesis;
end;
