Ta có:\(\dfrac{20}{2\times7}+\dfrac{20}{7\times12}+\dfrac{20}{12\times17}+\dfrac{20}{17\times22}+\dfrac{20}{22\times27}+\dfrac{20}{27\times32}\)

 \(=4\times\left(\dfrac{5}{2\times7}+\dfrac{5}{7\times12}+\dfrac{5}{12\times17}+\dfrac{5}{17\times22}+\dfrac{5}{22\times27}+\dfrac{5}{27\times32}\right)\)

    \(=4\times\left(\dfrac{1}{2}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{12}+\dfrac{1}{12}-\dfrac{1}{17}+\dfrac{1}{17}-\dfrac{1}{22}+\dfrac{1}{22}-\dfrac{1}{27}+\dfrac{1}{27}-\dfrac{1}{32}\right)\)

\(=4\times\left(\dfrac{1}{2}-\dfrac{1}{32}\right)=4\times\dfrac{15}{32}=\dfrac{30}{16}\)

1. They sell jeans all over the world. 

=> Jeans are sold all over the world

2. They are building a new school in this town. 

=> A new school are being built in this town

3. The clown made us laugh a lot. 

=> We were made to laugh a lot by the clown

4. He finished his exercises on time.

=> His exerxises were finished on time by him

5. People plant rice in those fields.

=> Rice  in those fields is planted

6. this boy broke my windows yesterday.

=> My windows were broken yesterday by this boy

7. did you do your test perfectly?

=> Was your test done perfectly ?

8. They have known him well.

=> He has been known well

9. They have postponed the concert.

=> The concert has been postponed 

10. My father bought me a video recorder.

=> I was bought a video recorder by my father

11. The police didn’t interview the witness.

=> The witness weren't interviewed by the police

12. They use cars and trucks to carry food to market.

=> Cars and trucks are used to carry food to market

13. They have built a new hospital near the airport.

=> A new hospital has been built near the airport

14. He hasn’t seen the younger son for three weeks.

=> The younger son hasn't been seen for three weeks by him

15. The secretary opens the mails every morning.

=> The emails are opened every morning by the secretary

16. No one has opened that box for the past hundred years.

=> That box hasn't been opened for the past hundred years

17. The fire has destroyed many houses.

=> Many houses have been destroyed by the fire

18. The children are carrying the chairs into the house.

=> The chairs are being carried into the house by the children

19. She cleans the floor every morning

=> The floor is cleaned every morning by her

20. He will solve the problem soon.

=> The problem will be solved soon by him

21. She made a lot of mistakes in the examination.

=> A lot of mistakes were made in the examination by her

22. No one has locked the door.

=> The door hasn't been locked

1.B

2.B

3.D

4.A

5.D

6.B

7.A

8.C

9.B

10.B

11.C

12.B

13.D

14.C

15.B

Ta có:\(\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}+\dfrac{1}{64}\)

     \(=\dfrac{32}{64}+\dfrac{16}{64}+\dfrac{8}{64}+\dfrac{4}{64}+\dfrac{2}{64}+\dfrac{1}{64}=\dfrac{63}{64}\)

Ta có:\(\dfrac{5083}{2431}=\dfrac{23}{11}\)

a)nữ sĩ

b)nữ giới

B.32 gam

có nửa chu vi rồi mà, chia ra nữa thành luôn 1/4 chu vi 

đề đâu?

2-3=-3 ?