Ta có \(n\left(n+3\right)\left(n+6\right)\left(n+9\right)+81\)

\(=n\left(n+9\right)\left(n+3\right)\left(n+6\right)+81\)

\(=\left(n^2+9n\right)\left(n^2+9n+18\right)+81\)

\(=\left(n^2+9n+9-9\right)\left(n^2+9n+9+9\right)+81\)

\(=\left(n^2+9n+9\right)^2-9^2+81\)

\(=\left(n^2+9n+9\right)^2\) 

\(\Rightarrow n\left(n+3\right)\left(n+6\right)\left(n+9\right)+81\) là scp 

thiếu đề r bạn \(a^2+b^2\ge\) 

this river is very ( danger / dangerously / dangerous) for the swimmers
walking is a good sports ( game / acctivity / health ).
It makes us ( healthy / activist / health )
I (must / can / shouldn't ) stay in bed because I have a bad cold
I like swimming ( so do / yes, I do / I do, so ) câu này là so do I

một thế giới kỳ diệu

9. The assistant told me to leave my address with the secretary

10. Jame asked me If I wanted him to send that letter for me

11. People built that theatre two years ago, didn't they?

12. It's surprising for us to get the scholarships

13. Becasue of the heavy rain, I couldn't go to school

14. Becaise my friend is absent, I have to copy the lesson for him

15. I'd rather you kept quiet in the classroom

traditional không có s

1. attractions

2. products

3. traditionals

4. national

5. delighted

1.
There are both many good things and many potential dangers in social network 
It's important for parents to teach their children how to use social media wisely
Social network is the start of bad things like cyberbullying

2.

A recent report has said that many teenagers have contacted online with strangers and it makes them feel scared or uncomfortable.

Others receive onine advertising that is inapproriate for their age.

It is important that parents should be aware of what children are doing online.

Parents should make their children understand that they respect children's privacy.
However, parents want to make sure that their children are safe.

1. Miss Hoa is very ­­­­­­­­­­­­­­________________ . She goes to church every morning.

A. religious                B.  religiousl         C. religion         D. religioner 

2. The hospital building is divided_______________ four sections.

A. in                             B. into                 C. to                  D. about

3. Did he  _____________ live in the country when he was young?.

A. get used to              B. used to             C. use to                D. be used to

4. In my school, we can choose between art and music. They are _______________.

A. optional         B. interesting             C. independent            D. compulsory

5. Minh usually wears uniform at school. At home he wears __________________ clothes.

A.  occasional              B. formal            C. casual                 D. casually

6. Tourists were really ____________ by the unique oil painting collections

A. impress           B. impression           C.  impressed         D. impressing

7. I hope to see you again.  “   “ _____________”.

A.I hope so, too.          B. Good enough         C. Thank you        D. I think so.

8. Their religion is Buddhism, so they go to the ____________ to pray every day.

A. country               B. pagoda            C. mosque                   D. church

9.     Maryam and Lan ____________ each other for over two years now.
A. know          B. knew             C. have known       D. are knowing