Câu 9:

- Đơn chất đc tạo từ 1 nguyên tố
- Hợp chất là tạo từ 2 nguyên tố trở lên

Câu 10: Chất đều do phân tử tạo thành, phân tử gồm những nguyên tử cùng loại là đơn chất, phân tử gồm những nguyên tử khác loại là hợp chất. 

Question 27. "I never make mistakes," he said. → He said____he never made mistakes.____

Question 28. Let’s have a walk in the woods this afternoon. → How about ___having a walk in the woods this afternoon?___

Question 29. Her husband hated holidays so much. She decided to go on her own. (SINCE) → Since her husband hated holidays so much, she decided to go on her own.

Question 30. This boy told me, “My brother has played football for 5 years”. → This boy told ____me that his brother had played football for 5 years.___

Exercise 26
1. Mr Brown just (finish) had just finished reading the letter when the telephone on his desk (ring) rang.
2. He (come) came into the room a moment after I (get) had got there.
3. She was breathing fast and deep, as if she (run) had been running.
4. I hardly (be) have hardly been there 5 minutes when Mrs Brown (come) come in with the coffee.
5. Scarely they (move) had moved into the house when their friends (come) came in.
6. I (meet) met John before I (be) had been a fornight in Paris.
7. After the telephone (buzz) had buzzed for along while, the doctor (answer) answer it.
8. As soon as the boys (enter) entered the hall they (begin) began to speak about books and writers.
9. Roy (wait) will wait until the door (close) closes behind him.
10. I wish I (know) knew you needed the book. I (can buy) could buy it for you in London.

Exercise 25
1. I think he is the nicest person I (ever meet) have ever met.
2. The man at the corner table (glance) glanced my way to see if I (listen) was listening.
3. There (be) was a curious expression on his face I have never (see) seen before.
4. Before the war he (be) was a teacher at the University, as his father (be) was before him.
5. We (catch) would have catched the last bus if we (leave) had left the theatre 5 minutes earlier.
6. It (be) was one of the happiest afternoons he has never (spend) spent.
7. And then, turning the corner of the path, he (see) saw Mary exactly where he (see) had seen her the first time.
8. And, paying for what he (not eat) hadn't eaten, he (go) went out, passing two acquaintance without sign of recognition.
9. She had just (open) opened the letter a few moments before my call (come) came through.
10. I wish I (see) had seen more of him before the war.

Exercise 24
1. When I (look) looked at my suitcase, I could see that somebody (try) was trying to open it.
2. When we (get) got to the restaurant, we (find) found that nobody (remember) had remembered to reserve a table.
3. The doctor (exmine) examined him, and (find) found that he (break) had broken his arm.
4. I could tell at a glance that the pile of letters on my desk (disturb) had been disturbed while I (be) was out.
5. We (be) were a few minutes late, so the film (already start) has already started when we (get) got to the cinema.
6. As soon as she arrived England, she (find) found that the language (be) was quite different from the English that she (learn) had learnt at school.
7. "Good afternoon. Can I help you?" "Yes, I (bring) brought my watch to you for repair three weeks ago. Is it ready yet?".
8. I wanted to be the first to tell her news, but I was too late. Someone (already tell) had already told her.
9. The children are filthy. Where have they (be) been?
10. I'm going to bed. I (work) have been working for hours and I'm tired.

Exercise 23
1. You see, I (do) have done this kind of thing for the last 30 years.
2. Go on the next test as soon as you (finish) finish this one.
3. If you (go) go away you (write) will write to me, won't you?
4. There are mice in you kitchen. If you (have) have a cat, the mice (disappear) will disappear.
5. She was badly hurt when her car hit another car. If she (wear) wears her seat belt, she (not hurt) won't hurt so badly.
6. Did the doctor recommend that you (stop) should stop smoking?
7. Is it important that we (be) will be on time for the meeting tonight?
8. I feel as if my head (be) were on fire now, doctor.
9. He talked as though he (know) had known where she was.
10. When he (start) started talking. I (realise) realised that we (meet) had meet before.

7⁰]

Bài 6:

Bài 5: 

Xét ΔABC vuông tại A

Áp dụng Pytago ta có:

⇒

DE là trung trực của BC

⇒ E là trung điểm của BC; DE vuông góc với BC tại E

⇒ EC = BC/2 = 40/2 = 20

Xét ΔCED và ΔCAB có:

∠CED = ∠CAB = 90^o

∠C chung

⇒ ΔCED đồng dạng ΔCAB

⇒ CE/CA = ED/AB

⇒ 12/32 = ED/24

⇒ ED = 9