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19.

\(\left(a+b\right)^2\le2\left(a^2+b^2\right)=4\Rightarrow-2\le a+b\le2\)

\(P=3\left(a+b\right)+ab=3\left(a+b\right)+\dfrac{\left(a+b\right)^2-\left(a^2+b^2\right)}{2}=\dfrac{1}{2}\left(a+b\right)^2+3\left(a+b\right)-1\)

Đặt \(a+b=x\Rightarrow-2\le x\le2\)

\(P=\dfrac{1}{2}x^2+3x-1=\dfrac{1}{2}\left(x+2\right)\left(x+4\right)-5\ge-5\) (đpcm)

Dấu "=" xảy ra khi \(x=-2\) hay \(a=b=-1\)

20.

Đặt \(P=2a+2ab+abc\)

\(P=2a+ab\left(2+c\right)\le2a+\dfrac{a}{4}\left(b+2+c\right)^2=2a+\dfrac{a}{4}\left(7-a\right)^2\)

\(P\le\dfrac{1}{4}\left(a^3-14a^2+57a-72\right)+18=18-\dfrac{1}{4}\left(8-a\right)\left(a-3\right)^2\le18\) (đpcm)

Dấu "=" xảy ra khi \(\left(a;b;c\right)=\left(3;2;0\right)\)

\(=\dfrac{\sqrt{x}\left(\sqrt{x}-1\right)}{\sqrt{x}-1}+\dfrac{\sqrt{x}\left(\sqrt{x}+1\right)}{\sqrt{x}+1}\)

\(=\sqrt{x}+\sqrt{x}=2\sqrt{x}\)

IV. Circle the best option to complete each sentence:

1. She usually _____ two hours a day

a. work          b. is working         c. works     d. working

2. Don’t make noise. My mother _________ with her friends

a. is talking         b. was talking        c. talks         d. talked

3. Listen! They _________ about the exhibition . Let’s join them

a. talk          b. talked        c. are talking        d. have talked

4. Where _____ your sister born?

a. were          b. was           c. did           d. is

5. We _____ have white wine, but we _____ red wine

a. didn’t/had         b. don’t/had     c. did/didn’t         d. didn’t/have

6. When did you _________ see him?

a. recently      b. already       c. last          d. lately

7. I _________ a bath when the phone rang

a. was having         b. had           c. am having     d. have

8. I _____ her two week ago

a. seen          b. was seeing         c. saw           d. have seen

9. I _____ to his house in 1991

a. moved     b. have been moving  c. have moved         d. was moving

10. He came into the room while they _________ television

a. have watched  b. watched       c. were watching d. have been watching

11. I fell asleep when I _____ television

a. am watching   b. am being watched  c. was watching d. All are correct

12. They _____________ last week

a. didn’t come   b. came not      c. don’t come         d. did come

13. Where are Geoff and Anne? _____________ in the garden

a. They’re sitting   b. They sitting       c. There sitting d. They’re siting

14. It’s very cold today and _____

a. It’s snowing       b. it snows      c. its snowing        d. it snowing

15. She goes to the library twice a week. She _____________ read books there

a. never     b. usually       c. rarely        d. once

16. At nine o’clock yesterday morning, we _____________ for the bus

a. are waiting        b. waited        c. were waiting  d. was waiting

17. Tom _________ his hand when he was cooking the dinner

a. burnt     b. was burning        c. has burnt     d. had burnt

18. He _________ me every Sunday, and we often go fishing together

a. came to see        b. comes to see  c. will be coming to seed. All answer are correct

19. The phone rang while she _________ dinner

a. cooks     b. was cooking   c. is cooking         d. has cooked

20. My sister _________ trouble with her motobike now, so she has to take the bus to work

a. have          b. is having     c. was having         d. had

21. When my mum got home, _________

a. the dinner be cooked           b. I was cooking dinner

c. I am cooking dinner             d. the dinner deing cooked

22. Emma and Katie usually _________ their grandmother at weekends

a. have visited       b. visits        c. visit         d. are visiting

23. Peter was reading a book by Mark Twain _________ his friends came

a. while     b. when      c. during        d. if

24. Look! That boy _________ an elephant over there

a. fed           b. was feeding        c. feeds     d. is feeding

25. He _________ in London at the moment

a. studies       b. has studied        c. is studying        d. will study

nguồn :hoidap247

1. C. works

2. A. is talking

3. C. are talking

4. B. was

5. A. didn’t/had

6. C. last

7. A. was having

8. C. saw

9. A. moved

10. C. were watching

11. C. was watching

12. A. didn’t come

13. A. They’re sitting

14. A. It’s snowing

15. B. usually

16. C. were waiting

17. A. burnt

18. B. comes to see

19. B. was cooking

20. B. is having

21. B. I was cooking dinner

22. C. visit

23. B. when

24. D. is feeding

25. C. is studying

TH1: thể 3 ở A, B, E

A-B-ddE- = 3.3.2.1.2 = 36

TH2: Thể 3 ở d

A-B-ddE- = 2.2.1.2 = 8

Tổng là 44

3 . ( 2x - 1 ) - 2 = 13 

3 . ( 2x - 1 ) = 12 + 3 

3 . ( 2x - 1 ) = 15 

      2x - 1 = 15 : 3 

      2x - 1 = 5 

     2x = 5 + 1 = 6 

x = 6 : 2 = 3 

Vậy x = 3

\(3\left(2x-1\right)-2=13\)

\(3\left(2x-1\right)=15\)

\(2x-1=5\)

\(2x=6\)

\(x=3\)

3.(2x - 1 ) - 2 = 13

\(\Leftrightarrow\) 3.( 2x - 1) = 15

\(\Leftrightarrow\) 2x- 1 = 15 : 3

\(\Leftrightarrow\) 2x-1 = 5

\(\Leftrightarrow\)2x = 6

\(\Leftrightarrow\) x = 3

Vậy x = 3

Câu nèo thé ?_?

Câu nào thế bạn????

where is câu hỏi???

between nha.

theo mình là between nha