Bài 1:

: a + b - 2c = 0

⇒ a² + b² + ab - 3c² = 0 ta có:

⇔ 

⇔ 

⇔ 

⇔ 

⇔ 

⇒ 

⇔ 

⇔ 

⇒  

Vậy 

a) Đường cao BH = CK = a

BC = a/sinα

Kẻ đg cao AD ⇒ BD = DC = a/2sinα

⇒ AD = BD.tanα = sinα/cosα . a/2sinα = a/2cosα

    AB = AC = AD/sinα = a/2sinαcosα = a/sin2α

b) Dễ dàng có đc S = pr

⇒ r = S/p = AD.BC/2AB+BC = a/2+2cosα

S = AB.BC.CA/4R

⇒ R = AB.BC.CA/4S = a/2sin22α.cosα

1. C

2. B

3. A

4. B

5. D

1. Everyone knows that taking exerice is good for your health. 

Taking exercise is good for your health.

2. The respone to our appeal was so great that we had to take on more staff. 

So great was the response to our appeal that we have to take on more staff.

3. “No, I really don’t want to stay the night at your house, Sophia,” Ann said.

Ann insisted on not staying the night.

 4. As soon as I got home, I realized I’d left my bag in the shop.

No sooner had I got home than I realised I'd left my bag in the shops. 

5. “I’ll call off the football match if you don’t behave,” the teacher said to the children.

The teacher threatened that he will call of the football match unless the children's behaviour improved.

6. The demand for tickets was so great that people queued day and night.

Such was the demand for the tickets that people queued day and night.

7. “Congratulations on getting engaged, Sue,” said Harry.

Harry congratulated Sue on getting engaged.

8. It was only when I stopped that I realized something was wrong. 

Only when I stopped did I realize that something wrong.

9. “I wouldn’t stay out in the cold for too long,” Jill told Tom.

Jill advised Tom not stay out in the cold for too long. 

10. “Why don’t you come round to my house for a meal,” Alan told me. 

Alan invited me come round to his house for a meal.

31. D. on

32. A. computer

33. C. mind

34. B. for

35. C. hobby 

36. D. real

37. A. lead

38. D. addicted

5. Western

6. started

7. Europe

8. continued

9. was

10. began

5. Western

6. started

7. Europe

8. continued

9. was

10. began

Ta có: ∠∠∠∠

∠∠

∠∠

Lại có: ∠∠

∠∠

∠

∠

∠∠

∠D = 100^o/2 = 50^o

1. B

3. C