MỞ XUỐNG THÌ TRẢ LỜI CÂU KHÓ!!!!!!!!!!!!!!!!!!!!!!!!
999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 x 99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999=
chịu !!!!!!!!!!!!!!!!!!!!!!!!!!!!
1 số nào đó thuộc tập hợp N và chắc chắn số đó >0