Nowadays, jeans are fashionable and familiar to almost everyone. Especially for the young generation, jeans are recommended because of their benefits in the economy. First of all, unlike normal pants which could be worn out by us doing strong activities, throughout time, etc; jeans are much more durable and could last for a long time. Moreover, jeans are popular and suitable for everyone due to their variety in styles, sizes and prices; therefore jeans could be cheap and affordable for all social classes. Jeans are also compatible with many kinds of clothes such as T-shirts, shirts, blouses. Last but not least, jeans have never been out of fashion since it was introduced which can save us a lot of time and money, not having to be slaving over fashion trends.

27/ He used to play football for the local team, but he is too old now.

28/ She would travel all over the world if she was rich

29/ Beth wishes that she could stay at Cambridge forever

30/ Every country has its own customs and technological changes.

2Ca+ O₂ → 2CaO

(1)CaO+ H₂O→ Ca(OH)₂

(2) Ca + 3H₂O→ Ca(OH)₂+2H₂O

(1) Ca(OH)₂+ H₂CO₃→ CaCO₃+2H₂O 

(2) CaO+CO₂→CaCO₃

CaCO₃→CaO+ CO₂ (phân hủy)

CaO+2HCl→CaCl₂+H₂O

S+O₂→SO₂

SO₂+Na₂O→Na₂SO₃

Na₂SO₃→SO₂+Na₂O

(1)SO₂+O₂→H₂SO₃

(2) SO₂+BaO→BaSO₃

(x + 1) + (x + 2) + (x + 3) + ...... + (x + 9) = 90

x + 1 + x + 2 + x + 3 + .... + x + 9 = 90

(x + x + x + .... + x) + (1 + 2 + 3 + ..... + 9) = 90

9x + (1 + 2 + 3 + ..... + 9) = 90

Áp dụng công thức tính dãy số ta có :

\(\frac{\left[\left(9-1\right):1+1\right].\left(9+1\right)}{2}=\frac{9.10}{2}=45\)

=> 9x + 45 = 90

=> 9x = 90 - 45 = 45

=> 9x = 45

=> x = 45 : 9 = 5

106 kg đó bn chắc 100% mk thi rùi

k mk nha

34,54m đấy.Mình thi vio đúng rồi

1 - 2 + 3 - 4 + ..... + 99 - 100

= 1 + (-2) + 3 + (-4) + ..... + 99 + (-100)

Số các số hạng là : 100

1 + (-2) + 3 + (-4) + ..... + 99 + (-100)

= [1 + (-2)] + [3 + (-4)] + ..... + [99 + (-100)]

Vì mỗi cặp có 2 số hạng mà có tất cả 100 số 

=> Có tất cả 100 : 2 = 50 cặp

[1 + (-2)] + [3 + (-4)] + ..... + [99 + (-100)]

= (-1) + (-1) + ..... + (-1)

= (-1) . 50

= -50

4x + 3 chia hết cho x - 2

4x - 8 + 8 + 3 chia hết cho x - 2

4.(X - 2) + 11 chia hết cho x - 2

Vì (x - 2) chia hết cho x - 2

=> 4(x - 2) chia hết cho x - 2

=> 11 chia hết cho x - 2

=> x - 2 thuộc Ư(11) = {1 ; -1 ; 11; -11}

Ta có bảng sau :

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