Gọi độ dài cả quãng đường là S.

Trong nửa đoạn đường đầu:

\(t_1=\dfrac{S_1}{v_1}=\dfrac{\dfrac{1}{2}S}{v_1}=\dfrac{S}{2\cdot120}=\dfrac{S}{240}\left(h\right)\)

Trong nửa đoạn đường sau:

Trong nửa thời gian đầu:

\(t_3=\dfrac{1}{2}t_2=\dfrac{1}{2}\cdot\dfrac{S_2}{v_3}=\dfrac{1}{2}\cdot\dfrac{\dfrac{1}{2}S}{v_3}=\dfrac{S}{4\cdot80}=\dfrac{S}{320}\left(h\right)\)

Trong nửa thời gian còn lại:

\(t_4=\dfrac{1}{2}t_2=\dfrac{1}{2}\cdot\dfrac{S_2}{v_4}=\dfrac{1}{2}\cdot\dfrac{\dfrac{1}{2}S}{v_4}=\dfrac{S}{4\cdot40}=\dfrac{S}{160}\left(h\right)\)

\(\Rightarrow\Sigma t_2=t_3+t_4=\dfrac{S}{320}+\dfrac{S}{160}=\dfrac{3S}{320}\left(h\right)\)

Vận tốc trung bình trên cả quãng đường:

\(v_{tb}=\dfrac{S_1+S_2}{t_1+t_2}=\dfrac{S}{\dfrac{S}{240}+\dfrac{3S}{320}}=\dfrac{S}{\dfrac{960S}{13}}=0,0135\)km/h

`@` `\text {Ans}`

`\downarrow`

`63 \div (3x) = 3`

`3x = 63 \div 3`

`3x = 21`

`x = 21 \div 3`

`x = 7`

Vậy, `x = 7.`

3x=63:3

3x=21

x=21:3

x=7

vậy x=7

ngu

We found the final contestant's performance interesting

h)2x^2+5x-3

=2x^2-x+6x-3

=(2x^2-x)+(6x-3)

=x(2x-1)+3(2x-1)

=(2x-1)(x+3)

d)3x^2-8x+4

=3x^2-2x-6x+4

=(3x^2-2x)-(6x-4)

=x(3x-2)-2(3x-2)

=(x-2)(3x-2)

XI. Em hãy dùng từ gợi ý để viết hoàn chỉnh mỗi câu sau:

1.It's easy to find a book either with author life.

2.Those book at the back of the library are in Vietnamese.

3.Is your father watching TV in the sitting room at the moment?

4.Magazines and newspapers are on the rack at the corner on the left.

5.Our library is one of the largest libraries in our city.

XII. Em hãy dùng từ gợi ý để viết hoàn chỉnh những câu sau:

1.Trang is the best student in our class.

2.The refrigerator is next to the closet and in front of the dishwasher.

3.Traveling by train is shower but more interesting than by plane.

4.Miss Hue takes care of sick children in the same hospital as mother.

5.Which is the most suitable house for Mr.Lam and his family?

6.Uncle is a farmer and he grows a lot of vegetables in his farm?

7.It is difficult to find an apartment in Ho Chi Minh City?

8.Brazin is the most successful for soccer team in the world.

9. Who is the fattest person in your group?

10.Lessons in this year are longer and more difficult than those last year.

XIII. Em hãy sắp xếp những từ ngữ dưới đây thành câu có nghĩa:

1. How far is it from Trang'house to the market?

2. He lives on Hoang Quoc Viet street with his grandparents.

3. My class doesn't have many old students.

4. Who is the boy talking the Mrs.Quyen?

5. Hoa'old school is smaller than her new one.

6.Mrs.Hoang goes to school by bus every day.

7. How is Minh's new house different from his old one?

8. Nguyet is unhappy because she misses her parents.

9. Does Nhung have a lot of friends in her town?

10. It's twelve o'clock and we're going to the luch now.

Thế năng trọng trường:

- Quả dừa trên cây

- Chim đậu bên vách đá

- Bong bóng mắc kẹt trên cây

Thế năng đàn hồi:

- Lò xo bị co dãn

- Quả bóng bị móp 

- Chai nhựa bị biến dạng

e: \(E=\dfrac{1}{4}+\dfrac{1}{5}-\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{28}+\dfrac{1}{29}-\dfrac{1}{29}-\dfrac{1}{30}\)

=1/4-1/30

=15/60-2/60=13/60

c: =1/2-1/3+1/3-1/4+...+1/29-1/30=1/2-1/30

=15/30-1/30=14/30=7/15

d: =1/3-1/5+1/5-1/7+...+1/99-1/101

=1/3-1/101

=98/303