8. Cho An = 1 - 5 + 9 - 13 + 17 - 21 + 25 -... (n số hạng, giá trị tuyệt đối của số hạng sau lớn hơn giá trị tuyệt đối của số hạng trước 4 đơn vị, các dấu cộng và trừ xen kẽ).

a) Tính giá trị của A khi n = 20.

b) Tính giá trị của A theo n. 

Gíup mình giải bài này với cảm ơn các bạn nhiều!

x= 4211

9. is/ tells.

10. are.

11. has.

12. isn't.

B3

1. is.

2. teaches.

3. learn.

4. comes.

5. has.

6. goes.

7. meets.

8. drinks.

B4

1. plays.

2. drink.

3. starts.

4. live.

5. do.

6. speak.

7. watches.

B5

1. does -> do.

2. Do -> Does.

3. watch -> watches.

4. goes -> go.

5. carries -> carry.

6. don't -> doesn't.

7. plays -> play.

8. are -> is.

1. I can't swim as far as Jane can.
2. Linh did the test more quickly than we did.
3. Tim runs faster than Peter can.
4.I cannot speak French as fluently as John does.
5. Thuy speaks French more fluently than Phong does.
6. I don't play football as well as he does.
7. Mrs. Hoa's husband works harder than she does.
8. We didn't answer the teacher's question yesterday as intelligently as they did.
9. Hai's friend listened to the teacher yesterday more attentively than Hai could.
10. I don't run as fast as she does.
11. Last year, Thanh didn't work as hard as he does now.
12. Last year, he understood the lessons better than he does now.
13. I usually get up later than my sister does.
14. Yesterday, Mr. Withe drank more than Mr. Pike did.
15. Tom is a more persuasive speaker than Bill.
16. She knows more about it than I do.
17. La drives more slowly and carefully than I do.
18. Robert can't cook as well as Jane does.
19. He arrived later than we expected.

x = 23 30 

x = − 43 30

9h

4. M = -3

6.

a = 2

b = -13

a) -6

b) 4