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Bài 1 :
\(S=\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{2009.2010.2011}\)
\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{2009.2010}-\frac{1}{2010.2011}\)
\(=\frac{1}{1.2}-\frac{1}{2010.2011}<\frac{1}{2}\)
Vậy \(S<\frac{1}{2}\)
Bài 2:
Làm nhiều rồi vào trong chỗ góc học tập của tớ mà coi 
I.Chọn đáp án đúng ( Các em viết câu và điền luôn đáp án)
1. My brother and I .............television every evening.
A. watches B. listen C. are D. watch
2. There is a well ................Minh’s house.
A. behind B. next C. near to D. to the left
3. A paddy-field is a rice .......................... .
A. yard B. garden C. paddy D. park
4. They are good students. ...................books are new.
A. They B. Their C. These D. Those
5. Ba ..................lunch at school .
A. have B. has C. eat D. does
6. What time ...........................classes start?
A. do B. does C. is D. are
7. Lan’s brother ....................volleyball now.
A. is play B. are playing C. is playing D. playing
8. This is my father . .............. ..name’s Ha.
A. His B. He’s C. She’s D. Her
9. I go to school .................ten past twelve.
A. in B. on C. at D. for
10. This is his sister. ...................... is a doctor .
A. He B. She C. It D. Her
11. Mr Ba travels to work .....................motorbike.
A. at B. for C. in D. by
12. How many ..........................are there in the classroom ?
A. table B. benches C . board D. desk
13. There is .......................... fruit juice in the pitcher .
A. some B. any C. a D. an
14. His lips aren’t full. They are ................................. .
A. fat B. weak C. thin D. long
15. We would like some vegetables ...........................dinner.
A. in B. at C. for D. of
Việt và Nam đang làm bài tập về nhà môn Tiếng Anh. Đề như sau:
1. Odd-one-out
A. the B. a C. an D. one
Việt nói: "Đáp án A là đúng vì a;an;one có nghĩa là một, còn the thì không phải." Nam nói:" Sai rồi. Đáp án D mới là đáp án đúng vì the;a;an đều là mạo từ, còn one thì không phải." Vậy bạn nào đúng, bạn nào sai? Tại sao?
Bạn Nam nói đúng. Vì a,an,the là mạo từ, one là số đếm với lại đề bài là odd one out.
Việt và Nam đang làm bài tập về nhà môn Tiếng Anh. Đề như sau:
1. Odd-one-out
A. the B. a C. an D. one
Việt nói: "Đáp án A là đúng vì a;an;one có nghĩa là một, còn the thì không phải." Nam nói:" Sai rồi. Đáp án D mới là đáp án đúng vì the;a;an đều là mạo từ, còn one thì không phải." Vậy bạn Nam đúng
I.Chọn đáp án đúng ( Các em viết câu và điền luôn đáp án)
1. My brother and I .............television every evening.
A. watches B. listen C. are D. watch
2. There is a well ................Minh’s house.
A. behind B. next C. near to D. to the left
3. A paddy-field is a rice ...........................
A. yard B. garden C. paddy D. park
4. They are good students. ...................books are new.
A. They B. Their C. These D. Those
5. Ba ..................lunch at school .
A. have B. has C. eat D. does
6. What time ...........................classes start?
A. do B. does C. is D. are
7. Lan’s brother ....................volleyball now.
A. is play B. are playing C. is playing D. playing
8. This is my father . .............. ..name’s Ha.
A. His B. He’s C. She’s D. Her
9. I go to school .................ten past twelve.
A. in B. on C. at D. for
10. This is his sister. ...................... is a doctor .
A. He B. She C. It D. Her
11. Mr Ba travels to work .....................motorbike.
A. at B. for C. in D. by
12. How many ..........................are there in the classroom ?
A. table B. benches C. board D. desk
13. There is .......................... fruit juice in the pitcher .
A. some B. any C. a D. an
14. His lips aren’t full. They are ................................. .
A. fat B. weak C. thin D. long
15. We would like some vegetables ...........................dinner.
A. in B. at C. for D. of
Viết lại câu giữ nguyên nghĩa ban đầu :
1. Hoa is tall.
Hoa isn’t ...short.........................................................
2. He often walks to school.
He often goes......to school on foot...............................................
3. She has an oval face .
Her ...face is oval...................................................................
4. My mother’s hair is long.
.......My mother has long hair................................................................
6. Lan ‘s eyes are brown.
Lan has .....brown eyes..................................................................
8. My class has thirty students .
There are ___thirty students in my class______________________________ .
9. My house is on the left of the bookstore .
The bookstore __is on the right of my house___________________________ .
10. There are twenty classrooms in Hoa’s school .
Hoa ‘s school __has twenty classrooms____________________________
11. What about taking some photos ?
Why __don't we take some photos _________________________________ ?
12. I want some chocolate .
I’d _like to have some chocolate_____________________________ .
13. The menu doesn’t have fish and eggs.
There __aren't fish and eggs in the menu____________________________ .
14. There are some boxes of chocolate in the store today
The store __has some boxes of chocolate today_______________________________ .
15. What about taking some photos ?
Why ....don't we take some photos..........................................................?
16. Are there four windows in her room ?
Does ...her room have four windows?..............................................................
17. Does your class have twenty-five students?
Are ...twenty-five students in your class...........................................................?
18. He goes to work at seven fifteen.
At a ....quarter past seven he goes to work........................................................
19. Mr Minh usually drives to work .
Mr Ming goes to…work by car…
20. How much is a bowl of noodles ?
How much does ....a bowl of noodles cost...............................................?
21. What is the price of a cake and an orange ?
How ....much is a cake and an orange ..............................................................?
22. How much does a box of chocolates cost ?
What ......is the price of a box of chocolate................................................?