reserve x,x1,x2,x3 for Real;

theorem
  (sin(x))|^3=(3*sin(x)-sin(3*x))/4
proof
  (3*sin(x)-sin(3*x))/4=(3*sin(x)-(-4*(sin(x))|^3+3*sin(x)))/4 by Th16
    .=(4*(sin(x))|^3)/4;
  hence thesis;
end;
