25. He started his career as a teacher 20 years ago.

→ He has worked as a teacher for 20 years
26. Is this your first visit to our country?

→Haven't you visited to our country before?
27. The last time we contacted was 3 months ago.
→ We haven't contacted for 3 months 
28. It is a long time since I last wrote to her.
→I haven't written to her for a long time
29. When did you start to wait for the bus?
→How long have you waited for the bus?
30. This is the second time I have stayed in this hotel.
→ I have stayed in this hotel once before
31. He last sent me postcards when he was in Paris.
→The last time..he sent me postcards was when he was in Paris

→He hasn't sent me postcards since he was in Paris
32. They haven't visited their hometown since they were 12.
→ They last ....visited their hometown when they were 12
33. This is the first time she has read such a romantic novel

. →She hasn't......read such a romantic novel before....
34. We began to talk two hours ago.

→ It's two hours...since we began to talk 
35. My father started to work as an engineer when he was 22.

→My father has worked as an engineer since he was 22
36. She last sent me letters last year.
→ She hasn't...sent me letters for a year....
→The last time....she sent me letters was last year.
37. The last time she phoned her parents was ages ago.
→ It's ages since she last phoned her parents
38. It is a long time since we last went to this 

→We haven't gone to this for a long time

39. When did you move to this house?

→ How long have you moved to this house?

40. She moved to this city 4 weeks ago

→ She has moved to this city for 4 weeks

13. We have chatted on the phone since it began to rain.

14.We have known each other since we were at university.

15.She hasn't phoned her parents for 3 weeks.

16.We haven't been to this restaurant for a long time.

17.How long have you had the laptop?

18.I haven't joined such an exciting party before.

20.The last time I heard from her was many years ago.

21.The last time the children were taken to the park was in 2012.

22.This is not our first visit to this city.

23.I have seen this movie twice.

24.He has been to a tennis match twice.

25.She has done the housework since she got home.

`sqrt{12,1 . 100}`

`= sqrt{121 . 10}`

`= sqrt{121} . sqrt{10}`

`= 11sqrt{10}`

``

`sqrt{2500 . 4,9 . 0,9}`

`= sqrt{25 . 10 . 10 . 4,9 . 0,9} `

`= sqrt{25 .49 . 9} `

`= sqrt{5^2 .7^2 . 3^2} `

`= 5 . 7 . 3 `

`= 105`

``

`sqrt{41^2 - 40^2}`

`= sqrt{(41+40)(41-40)}`

`= sqrt{81 }`

`= 9`

`sqrt{72} . sqrt{50}`

`= sqrt{36 .2} . sqrt{25.2}`

`= sqrt{6^2 .2} . sqrt{5^2 .2}`

`= 6 sqrt{2} . 5sqrt{2}`

`= 30 . 2 `

`= 60 `

``

`(sqrt{12} + sqrt{3}).(sqrt{27} - sqrt{3})`

`= (2sqrt{3} + sqrt{3}).(3sqrt{3} - sqrt{3})`

`= 3sqrt{3} . 2sqrt{3}`

`= 6 . 3 `

`= 18`

Do 2 bài trùng lặp nên mình làm tiện thể nhé.

`Mg+ZnSO_4->MgSO_4+Zn`

`2Al+3ZnSO_4->Al_2(SO_4)_3+3Zn`

Bài 2

PTHH:

`Mg+2HCl->MgCl_2+H_2`

0,1------------0,1---------0,1 mol

`n Mg=(2,4)/24=0,1 mol

=>`m_(MgCl_2)=0,1.95=9,5g`

=>`m_(H_2)=0,1.2=0,2g`

+, Trong 3 kim loại Mg, Al, Fe chỉ có duy nhất Fe không phản ứng với dung dịch ZnSO_4.
 +, PTPU: Mg + ZnSO₄ -> MgSO₄ + Zn
                 2Al   + 3ZnSO₄ -> Al₂(SO₄)₃ + 3Zn

`sqrt{54 - 18sqrt{5}}`

`= sqrt{54 - 2.3. 3sqrt{5}}`

`= sqrt{45 - 2.3. 3sqrt{5} + 9}`

`= sqrt{(3sqrt{5})^2 - 2.3. 3sqrt{5} + 3^2}`

`= sqrt{(3sqrt{5}- 3)^2}`

`= |3sqrt{5}- 3|`

`= 3sqrt{5}- 3`