đổi 24\(\dfrac{3}{5}\)m=14,4m; 18\(\dfrac{9}{20}\)phút=8,1phút

Thời gian Nhung đi từ A đến B là

14,4 chia 8,2 ≈ 1,8 phút

Thời gian Nhung đi từ B đến C là

8,1-1,8=6,3 phút

Quãng đường từ C đến B là

14,4-8,6=5,8m

Vận tốc của Nhung khi di chuyển từ B đến C là

5,8 chia 6,3 ≈ 0,92 m/phút

Để tìm hóa trị của nhóm nguyên tố PO₄ trong phân tử Ca₃(PO₄)₂, chúng ta cần hiểu một chút về cách các nguyên tử kết hợp với nhau trong các hợp chất.

Ca₃(PO₄)₂ là hợp chất bao gồm canxi (Ca) và nhóm photphat (PO₄). Trong hợp chất này:

·        Canxi (Ca) có hóa trị là 2+ (mỗi ion canxi mất 2 electron để trở thành Ca²⁺).

·        Chúng ta có 3 ion canxi (Ca²⁺), do đó tổng điện tích là 3 x 2 = 6+.

Trong hợp chất Ca₃(PO₄)₂, tổng điện tích dương phải cân bằng với tổng điện tích âm của nhóm PO₄. Vì có 2 nhóm PO₄ nên ta viết tổng điện tích của 2 nhóm này là -6. Do đó:

·        Mỗi nhóm PO₄ phải có điện tích là -3.

Vậy, hóa trị của nhóm PO₄ trong phân tử Ca₃(PO₄)₂ là 3-.

a hoặc d

II. Read the passage and choose the best option (A, B or C)  

Each country has many good people who take care of others. For example, some of students in the United States often spend many hours as volunteers in hospitals, orphanages or homes for the elderly. They read books to the people in these places, or they just visit them and play games with them or listen to their problems. Other young volunteers go and work in the homes of people who are sick or old. They paint, clean up, or repair their houses, do the shopping. For boys who don’t have fathers, there is an organization called Big Brothers. College students and other men take these boys to basketball games or on fishing trips and help them to get to know things that boys usually learn from their fathers. Each city has a number of clubs where boys and girls can go and play games. Some of these clubs show movies or hold short trips to the mountains, the beaches, museums, or other places of interest. Most of these clubs use a lot of students as volunteers because they are young enough to understand the problems of younger boys and girls. 

1. What do volunteers usually do to help those who are sick or old in their homes? 

A. They tell them stories and sing and dance for them. 

B. They do shopping, and repair or clean up their houses. 

C. They cook, sew, and wash their clothes. 

2. What do they help boys whose father do not live with them? 

A. To learn things that boys usually learn from their fathers. 

B. To get to know things about their fathers. 

C. To learn things about their fathers. 

3. Which activity is NOT available for the students at the clubs? 

A. playing games and. going to interest places                         B. watching films 

C. learning photography 

4. Why do they use many students as volunteers? -Because…………………………. 

A. they know how to do the work                                B. they have a lot of free time 

C. they can understand the problems of younger boys and girls 

5. Where don’t students often do volunteer work? 

A. hospitals and orphanages     B homes for the elderly                                       C. clubs 

II. Read the passage and choose the best option (A, B or C)  

Each country has many good people who take care of others. For example, some of students in the United States often spend many hours as volunteers in hospitals, orphanages or homes for the elderly. They read books to the people in these places, or they just visit them and play games with them or listen to their problems. Other young volunteers go and work in the homes of people who are sick or old. They paint, clean up, or repair their houses, do the shopping. For boys who don’t have fathers, there is an organization called Big Brothers. College students and other men take these boys to basketball games or on fishing trips and help them to get to know things that boys usually learn from their fathers. Each city has a number of clubs where boys and girls can go and play games. Some of these clubs show movies or hold short trips to the mountains, the beaches, museums, or other places of interest. Most of these clubs use a lot of students as volunteers because they are young enough to understand the problems of younger boys and girls. 

1. What do volunteers usually do to help those who are sick or old in their homes? 

A. They tell them stories and sing and dance for them. 

B. They do shopping, and repair or clean up their houses. 

C. They cook, sew, and wash their clothes. 

2. What do they help boys whose father do not live with them? 

A. To learn things that boys usually learn from their fathers. 

B. To get to know things about their fathers. 

C. To learn things about their fathers. 

3. Which activity is NOT available for the students at the clubs? 

A. playing games and. going to interest places                         B. watching films 

C. learning photography 

4. Why do they use many students as volunteers? -Because…………………………. 

A. they know how to do the work                                B. they have a lot of free time 

C. they can understand the problems of younger boys and girls 

5. Where don’t students often do volunteer work? 

A. hospitals and orphanages     B homes for the elderly                                       C. clubs 

Lời giải chi tiết:

Các thành tựu:

Kiến trúc:

- Ba loại hình: kiến trúc cung điện, kiến trúc tôn giáo, kiến trúc lăng tẩm

- Công trình: Tử Cấm Thành, chùa Thiên Ninh, Thập Tam lăng

Điêu khắc :

Phong phú về đề tài và chất liệu.

Công trình: tượng Phật nghìn mắt nghìn tay và tượng Phật trên núi Lạc Sơn

Hội họa :

tranh thủy mặc, thư pháp, họa pháp

Tác gia nổi tiếng: Triệu Mạnh Phủ, Tô Đông Pha,...

Văn học:

 Thơ Đường (3 tác giả nổi bật Lý Bạch, Đỗ Phủ, Bạch Cư Dị)

Tiểu thuyết: Thủy hử, Tây du ký, Hồng lâu mộng, Tam quốc diễn nghĩa

Sử ký của Tư Mã Thiên

Các bộ bách khoa: Vĩnh Lạc đại điển và Tứ Khố toàn thư .

\(\dfrac{x}{y}=\dfrac{y}{10}=\dfrac{10}{x}=\dfrac{x+y+10}{y+10+x}=1\)

\(\Rightarrow x=y=10\)

@Nguyễn Thị Thương Hoài

@Nguyễn Lê Phước Thịnh

@Lê Minh Vũ

.... Giúp em với ạ!!

\(\dfrac{9^5-27^3+81^2}{\left(-63\right)}\)

Ta có: \(9=3^2\Rightarrow9^5=\left(3^2\right)^5=3^{10}\)

\(27=3^3\Rightarrow27^3=\left(3^3\right)^3=3^9\)

\(81=3^4\Rightarrow81^2=\left(3^4\right)^2=3^8\)

\(=\dfrac{3^{10}-3^9+3^8}{\left(-63\right)}\)

\(=\dfrac{3^9}{\left(-63\right)}\)

\(=\dfrac{45927}{\left(-63\right)}\)

\(=-729\)

\(\dfrac{9^5-27^3+81^2}{-63}\)

\(=\dfrac{3^{10}-3^9+3^8}{-63}=\dfrac{3^8\left(3^2-3+1\right)}{-63}\)

\(=\dfrac{-3^8\cdot\left(9-3+1\right)}{63}=\dfrac{-3^8}{3^2\cdot7}\cdot7=-3^6\)

=-729

  \(\dfrac{1}{99}\) - \(\dfrac{1}{99.97}\) - \(\dfrac{1}{97.95}\) - .. - \(\dfrac{1}{3.1}\)

= \(\dfrac{1}{99}\) - (\(\dfrac{1}{99.97}+\dfrac{1}{95.93}+\dfrac{1}{3.1}\))

= \(\dfrac{1}{99}\) - \(\dfrac{1}{2}\) (\(\dfrac{2}{1.3}+\dfrac{2}{3.5}+..+\dfrac{2}{99.97}\))

= \(\dfrac{1}{99}\) -  \(\dfrac{1}{2}\)(\(\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{97}-\dfrac{1}{99}\))

= \(\dfrac{1}{99}\) - \(\dfrac{1}{2}.\) (\(\dfrac{1}{1}\) - \(\dfrac{1}{99}\))

= \(\dfrac{1}{99}\) - \(\dfrac{1}{2}.\)\(\)\(\dfrac{98}{99}\)

= \(\dfrac{1}{99}\) - \(\dfrac{49}{99}\)

=  - \(\dfrac{16}{33}\)