Happiness is only a state of mind and it is often associate with material prosperity. The poor think that happiness is the monopoly oi the rich, but the rich are not always happy. They have their problems and difficulties. If a poor man comes by a little wealth, he craves for more. Men always pine for what is not. A millionaitre won't consider his wealth to be sufficient. In the fact there is no end to man 's hankering after more of worldly goods. Happiness doesn't go hand with prosperity. 

  Material prosperity, however, is generally believed to lead to happiness in the world of today. He who is regarded as happy has no wants left unsatisfied, who has a score of servants to attend to his needs and whose wishes are fulfilled at once at his very bidding.

   Is it true that poverty and happiness go ill together. But there is no doubt that under certain conditions even the poorest man can make himself happy.

1) Is she playing badminton now? 

2) Are you doing your homework? 

3) Is Nam watching TV right now? 

4) Is your teacher teaching in that class? 

5) Are you and your friends going fishing at the moment? 

6)Are you hiding somewhere?

7) Are Lan and Mai playing computer game at this time? 

8) Is he swimming?

9) Are they going to the bookshop? 

10) Is Hoa phaying the piano now? 

Giả sử √2 là số hữu tỉ thì nó viết được dưới dạng :

m/n với m, n thuộc N, (m, n)=1

Do 2 không là số chính phương nên m/n không là số tự nhiên, do đó n>1

Ta có m²=2n². Gọi p là ước nguyên tố nào đó của n, thế thì m² chia hết cho p 

=> m chia hết cho p.

=> p là ước nguyên tố của m và n (trái với (m, n) =1)

=>√2 ko là số hữu tỉ 

=>√2 là số vô tỉ.

Căn 15 là số vô tỉ mà bạn