Dear Peter

I received your email last week, in which you asked me about my city Ha Noi in 2050. There are different ideas about this question. I thought about it in a few days, and now I can give you two predictions.

Let's take a look at optimis aspects. In 2050 Ha Noi will become a more liveable city for lots of reasons. The city authorities have built a very careful plan to promote advanced technology to make greener motorbikes to deal with air pollution. In addition, the city dwellers are encouraged to make use of public transport system to reduce traffic jam. The government will also grow more trees and encourage people to use solar panels.

On the contrary, pessimis people think that the city life will be a dangerous place to live in because the government cannot control the use of private cars and motorbikes. Besides, they don’t believe that citizens will use public transports because this system in our city are aging and not upgraded regularly. It also take time to find a bus and go to the destination in spite of very short distance. People's life will also continue to be threatened because of pollution such as air pollution by burning fossil fuels, emission from cars and motorbikes, noise pollution from constructing buildings and sounding of transportations. The effects of global warming have great impacts on the city dwellers, higher temperature in the summer and colder climate in te winter.

That's all about my answer to your question. I hope you will be satisfied with it, Peter.

Best wishes,

Nga

 k mình nha bn CHÚC BN HỌC TỐT

a.Hàm số xác định 
<=> sinx # 0
<=> x # kpi

Điều kiện xác định của hàm số đã cho là: 

\(1-sinx>0\Leftrightarrow sinx< 1\).

Ta có hàm \(sinx\le1\)nên điều kiện xác định tương đương với \(sinx\ne1\)

\(\Leftrightarrow x\ne\frac{\pi}{2}+k2\pi,\left(k\inℤ\right)\).

Tập xác định: \(D=ℝ\backslash\left\{\frac{\pi}{2}+k2\pi,k\inℤ\right\}\).

k mình

nha 

pls đúng

kick cho mình nha

\(\sin\left(\frac{2x}{3}-\frac{\pi}{3}\right)=0\)

\(\Leftrightarrow\frac{2x}{3}-\frac{\pi}{3}=k\pi\left(k\inℤ\right)\)

\(\Leftrightarrow\frac{2x}{3}=\frac{\pi}{3}+k\pi\left(k\inℤ\right)\)

\(\Leftrightarrow x=\frac{\pi}{2}+\frac{3k\pi}{2}\left(k\inℤ\right)\)

Đề là như thế này à bạn?

\(A^2_{2n}-C^3_{3n}=15n-4n^2\left(n\inℕ^∗\right)\)

\(\Rightarrow\frac{\left(2n\right)!}{\left(2n-2\right)!}-\frac{\left(3n\right)!}{3!\left(3n-3\right)}=15n-4n^2\)

\(\Rightarrow\left(2n-1\right)2n-\frac{\left(3n-2\right)\left(3n-1\right)3n}{6}=15n-4n^2\)

\(\Rightarrow6\left(4n^2-2n\right)-\left(9n^2-9n+2\right)3n=6\left(15n-4n^2\right)\)

\(\Rightarrow24n^2-12n-27n^3+27n^2-6n-90n+24n^2=0\)

\(\Rightarrow-27n^3+75n^2-108n=0\)(Vô nghiệm \(\forall n\inℕ^∗\))