a chia hết cho 3 và a chia hết cho 7

=> a chia hết cho 21 (vì 3 và 7 nguyên tố cùng nhau)

Các số chia hết cho 21 là: 0; 21; 42; ...........

Hình như đề bài của bạn có vấn đề rồi.

Uh, để mk coi lại

1 does - visit

visits

2 Does - go

goes

3 does - watch 

watches - doesn't watch

4 Do - like

don't like

1. Who does Sally visit in Spain?
  - She visits her aunt and uncle.
2. Does Tom go to school by bus?
  - No, he doesn't. He goes to school on foot.
3. What does she watch on TV?
  - She watches the news but she doesn't watch soap operas.
4. Do they like swimming?
  - Yes, they do. But they don't like running.
5. When do you go to a football match?
  - I go to a football match on Sundays.

1. Who does Sally visit in Spain?
  - She visits her aunt and uncle.
2. Does Tom go to school by bus?
  - No, he doesn't. He goes to school on foot.
3. What does she watch on TV?
  - She watches the news but she doesn't watch soap operas.
4. Do they like swimming?
  - Yes, they do. But they don't like running.
5. When do you go to a football match?
  - I go to a football match on Sundays

Phần mình tin đậm và in nghiêng là từ cần điền nhé. 

Giải thích vì sao ạ ^^

S=1/5^2+1/7^2+...+1/103^2

=>\(S< \dfrac{1}{5^2}+\dfrac{1}{5\cdot7}+\dfrac{1}{7\cdot9}+...+\dfrac{1}{101\cdot103}\)

=>\(S< \dfrac{1}{2\cdot5}+\dfrac{1}{2}\left(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{101}-\dfrac{1}{103}\right)\)

=>S<1/25+49/515<5/32

298; 300; 302; 304; 306

298; 300; 302; 304; 306

298; 300; 302; 304; 306 nha bn