\(u_k=u_{k-1}+4\left(k-1\right)+3=u_{k-2}+4\left(k-2\right)+4\left(k-1\right)+2.3=...\)

\(u_1+4\left(1+2+...+k-1\right)+3\left(k-1\right)=\left(2k+3\right)\left(k-1\right)\)

\(\Rightarrow lim\frac{\sqrt{u_{kn}}}{n}=lim\frac{\sqrt{\left(2km+3\right)\left(kn-1\right)}}{n}=k\sqrt{2}\)

Do đó :

\(\frac{a^{2019}+b}{c}=lim\frac{\sqrt{u_n}+\sqrt{u_{4n}}+\sqrt{u_{4^2n}}+...+\sqrt{u_{4^{2018}n}}}{\sqrt{u_n}+\sqrt{u_{2n}}+\sqrt{u_{2^2n}}+...+\sqrt{u_{2^{2018}n}}}\)

\(=lim\frac{\sqrt{2}\left(1+4+4^2+...+4^{2018}\right)}{\sqrt{2}\left(1+2+2^2+...+2^{2018}\right)}\)

\(=lim\frac{\frac{4^{2019}-1}{4-1}}{\frac{2^{2019}-1}{2-1}}=\frac{2^{2019}+1}{3}\)

\(\Rightarrow S=a+b-c=2+1-3=0\)

mình giải giúp bạn phần đầu phần sau bạn tự giải nha

Ta có \(SA\perp\left(ABC\right)\Rightarrow SA\perp BC;SA\perp AB\) Mà \(BC\perp AB\Rightarrow BC\perp\left(SAB\right)\Rightarrow BC\perp SB\) => tg SAB là tg vuông tại B

Xét tg vuông SAB có

\(SB=\sqrt{SA^2+AB^2}=\sqrt{4a^2+a^2}=a\sqrt{5}\)

\(\Rightarrow S_{SBC}=\frac{SB.BC}{2}=\frac{a\sqrt{5}.a\sqrt{3}}{2}=\frac{a^2\sqrt{15}}{2}\)

Trong mp(SBC) dựng \(NK\perp SB\)(K thuộc SB) mà \(BC\perp SB\) => NK//BC

Ta có NS=NC

=> NK là đường trung bình của \(\Delta SBC\Rightarrow NK=\frac{BC}{2}=\frac{a\sqrt{3}}{2}\)

Ta có \(2SM=MB\Rightarrow SM=\frac{MB}{2}\Rightarrow SM=\frac{SB}{3}=\frac{a\sqrt{5}}{3}\)

\(\Rightarrow S_{SMN}=\frac{SM.NK}{2}=\frac{1}{2}.\frac{a\sqrt{5}}{3}.\frac{a\sqrt{3}}{2}=\frac{a^2\sqrt{15}}{12}\)

\(\Rightarrow S_{MNBC}=S_{SBC}-S_{SMN}=\frac{a^2\sqrt{15}}{2}-\frac{a^2\sqrt{15}}{12}=\frac{5a^2\sqrt{15}}{12}\)

Trong mp(SAB) từ A dựng đường thẳng \(AH\perp SB\) (H thuộc SB)

Ta có \(BC\perp\left(SAB\right)\Rightarrow BC\perp AH\)

\(\Rightarrow AH\perp\left(SBC\right)\) => AH là đường cao của hình chóp A.MNBC

Xét tg vuông SAB có

\(AB^2=BH.SB\Rightarrow BH=\frac{AB^2}{SB}=\frac{a^2}{a\sqrt{5}}=\frac{a\sqrt{5}}{5}\)

Xét tg vuông ABH có

\(AH=\sqrt{AB^2-BH^2}=\sqrt{a^2-\frac{5a^2}{25}}=\frac{2a\sqrt{5}}{5}\)

\(\Rightarrow S_{A.MNBC}=\frac{1}{3}.S_{MNBC}.AH=\frac{1}{3}.\frac{5a^2\sqrt{15}}{12}.\frac{2a\sqrt{5}}{5}=\frac{5a^3\sqrt{3}}{18}\)

nguyễn ngọc anh đúng

* Paragraph 1: Habitat and location of the species below. (Đoạn 1: Môi trường sống và địa điểm của các loài được liệt kê bên dưới.)

Example:
- Komodo dragons are found in the wild... (Rồng Komodo được tìm thấy trong hoang dã ...)

- They live in forests,... (Chúng sống trong rừng, ...)

* Paragraph 2: Physical features and diet (Đặc điểm về hình thể và chế độ ăn uống)

Example:
- Although they are called dragons, they look ... (Mặc dù chúng được gọi là rồng nhưng chúng trông giống ...)

* Paragraph 3: Population and conservation status (Số lượng đàn và tình trạng bảo tồn)

Example:
- There are about 6,000... (Có khoảng 6.000 con ...)

-According to the IUCN Red List, Komodo dragons are classified as... (Theo sách đỏ của Tổ chức Bảo tồn Thiên nhiên Quốc tế, rồng Komodo được xếp vào....)
 

Blue whales are the largest animals to have ever existed.

(Các voi xanh là động vật lớn nhất từng tồn tại.)

They live in the cold waters of the Arc and Antarc but they migrate to tropical seas to breed.

(Chúng sống ở vùng nước lạnh ở Bắc Cực và Nam Cực nhưng chúng di cư đến vùng biển nhiệt đới để sinh sản.)

This breeding season lasts for about four months.

(Mùa sinh sản thường kéo dài tới bốn tháng.)

Blue whales are mammals but they look fish.

(Cá voi xanh là động vật có vú nhưng chúng nhìn giống như cá.)

They are 30 meters long and weigh about 150-200 tons.

(Chúng dài khoảng 30 mét và nặng khoảng 150-200 tấn.)

The earth's largest animal, the blue whale, usually eats very small shellfish and can eat 4 to 8 tons of krill per day.

(Là động vật lớn nhất trên trái đất nhưng cá voi xanh thường ăn các loại sinh vật có vỏ rất nhỏ và có thể ăn từ 4 tới 8 tấn nhuyễn thể mỗi ngày.)

A blue whale's tongue alone can weigh as much as an elephant — its heart as much as an automobile.

(Riêng lưỡi của cá voi xanh nặng bằng một con voi và trái tim của nó có thể nặng bằng một chiếc ô tô.)

There are about 14,000 blue whales in total (10,000 in the Antarc and 4,000 in the Arc).

(Có tổng số khoảng 14 nghìn con cá voi xanh (trong đó 10 nghìn con ở Nam Cực và 4 nghìn con ở Bắc Cực).)

According to the World Conservation Union (IUCN) Red List, blue whales are classified as endangered.

(Theo Sách đỏ của Cơ quan Bảo tồn Quốc tế, cá voi xanh được phân loại là có nguy cơ bị tuyệt chủng.)

Blue whales have few predators but are known to be victims in attacks by sharks and killer whales, and many are injured or die each year from collisions with large ships.

(Cá voi xanh có ít kẻ săn mồi nhưng chúng được cho là nạn nhân trong các đợt tấn công của cá mập và cá voi ăn thịt và nhiều con bị thương hoặc chết mỗi năm do va chạm với tàu lớn.)

Their lives are also badly affected by pollution and global warming.

(Cuộc sống của chúng cũng bị ảnh hưởng nghiêm trọng bởi ô nhiễm và hiện tượng trái đất nóng lên.)

In order to protect blue whales and stop them from declining, there are many recovery plans that have been launched.

(Để bảo vệ các voi xanh và ngăn chặn số lượng loài này bị suy giảm, có nhiều giải pháp khôi phục đã được đưa ra.)

I don't no 

sorry 

Can you viết again

Nowadays, people have many choices of commuting to work including public transport. However, it’s not always the best solution at all. Let’s consider its pros and cons.

It’s the fact that public transport has many benefits. Firstly, using bus, train,… save you an amount of money. Imagine you drive a private car or motorbike, you must pay for not only the cost of petrol but also the regular maintenance fee. Furthermore, the price of buying one car or motorbike is expensive itself. Bus, tram,… is for everyone, even the lowest-income people. Another advantage I want to mention is that using public vehicle is a method to reduce air pollution and traffic jams. Especially in the city, where the number of vehicles increasing makes the road crowded, the public transport system is developed to deal with traffic jam and reduce the pollutant released by petrol-run vehicles.

On the other hand, there are some disadvantages with public transportation. The problem that confuse people most is that it wastes time. Usually, people need to wait for some minutes to catch a bus a train, so they need to be punctually in order not to miss is. In some crowded cities, buses meet difficulties moving among so many vehicle, so people have to wait for quite a long time before the buses come, which annoy them and cause them to be late for schools or work. Besides, as I mentioned above, the fares are cheap so many people want to use it, sometimes it is crowded that it is impossible to breath and convenient for the pickpocket criminals to commit crime.

In conclusion, there are both positive and negative aspects of the using public transportation. Personally, I still believe that public transport is advantageous when the government build a modern public transport system to avoid the drawbacks.

1+1=2

1+2=3

1+3=4

Em viết phần intro nha:

The chart above decsribes how the birth rates of 2 countries namely China, USA changed in 80 years which started from 1920

Answer:
The line graph compares the birth rates in China and the US and how these rates changed from 1920 to 2000.
Overall, there was a general decline in fertility of both nations throughout the time surveyed. However, the US’s birth rates almost was higher than that of China over the whole period.
American saw a significant fluctuation in the birth rates between 1920 to 1935, but it always remained over 10%. However, in the following decade, the fertility of the US decreased sharply to under 5 %. From 1945 to 1950, there was a rapidly increased in American’s birth rates, at 15%, reaching the highest point of it’s own during the whole period. From that on, the fertility of the US fell gradually to 7% in 2000.
During the 80-year period, the birth rates in China changed more significantly than in the US. It fell down to 5% in the year 1940, then reaching a peak of whole line graph in 1950, at 20%. Over the remainder of the period, China saw a sharply drop in the birth rates to only about 2% until 2000.

Poverty is no shame

My parents died in a traffic accident when I was ten years old, so I grew up with my grandmother who was too old and weak to work. I had to leave school and did odd jobs to earn a living: selling lottery kets, washing dishes and doing babysitting. When I was 15, my grandma encouraged me to attend evening classes, so I could improve my literacy and job prospects.

After lots of hardship and effort, I completed secondary education and passed a challenging exam to enter Medical University, which is the dream of many students. However, I could not pay the tuition fees and had to reject the offer. The door to a new life was closed before me. I was completely disappointed and saw only gloomy days ahead.Then something incredible happened: I was awarded a scholarship from The Thanh Nien for college students from poor families. This was really a turning point in my life because I could start my university studies and hope for a better future.

Six years of hard work at the university and of trying to live on a very tight budget came to an end at last. Now I am working as a doctor at a hospital, but I will never forget those difficult days. I am planning to set up and run a charitable organisation to help poor students, as a way to acknowledge other people for supporting me.

I've told my own story hoping to encourage other people me to overcome hard times. Don't let poverty defeat you and destroy your hopes for a better future. This is my motto.

bằng 2 nha

bằng 2

Có Vchóp = 1/3 Sđáy.h = 1/3.a².2a =2/3a³

Có V. SAMN=V.SABD . 1/4=1/8V.SABCD=>.V.ABCDMN=7/8V.SABCD

=> V.ABCDMN = 7/8.2/3a³ = 7/12a³

7/12a³ nha