Khoảng thời gian khi về bằng 6/4 khoảng thời gian khi đi nên tổng thời gian đi và về là: 
1+6/4 = 5/2 (thời gian khi đi) 
Nếu đi với vận tốc 6km thì cả thời gian đi và về sẽ là 2 lần thời gian đi. 
Vì thời gian tỉ lệ nghịch với vân tốc nên vận tốc trung bình của người đó là: 
6 x 2 : 5/2 = 24/5 = 4,8 (km/giờ)

ĐS: 4,8 km/giờ

1. Tam Coc - Bich Dong, which belongs to Trang An Scenic Landscape Complex, is well known for its poe and inspirational natural scenery.

2. It is the second nicest cavern in Viet Nam, after Huong rich Cavern.

3. Bich Dong Pagoda was built in Le Dynasty in Bich Dong Cavern which/whose name means “Green Pearl Cavern”.

4. Tam Coc Cavern consists of Ca cave, Hai cave and Ba cave, all of which offer both beauty and mystery to tourists.

5. Visiting Tam Coc Cavern, tourists will feel  getting lost in such a hidden fairy site.

6. In brief, it is an ideal ecological spot for lovers of nature.

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1. Progress in science is being made day after day. → People are making progress in science day after day.

2. A French architect designed the palace.

3. They have paved the road in front of my house.

4. You can see that they haven't washed the dishes.

5. They will not increase our salaries this year.

6. Didn't they build that theatre two years ago?

7. They may discuss the problem again.

8. They have offered my brother a well – paid job.

9. People reported that the war started again in South American.

10. He told me that his football team had played well last season.

2. A French architect designed the palace.

3. They have paved the road in front of my house.

4. You can see that they haven't washed the dishes.

5. They will not increase our salaries this year.

6. Didn't they build that theatre two years ago?

7. They may discuss the problem again.

8. They have offered my brother a well – paid job.

9. People reported that the war started again in South American.

10. He told me that his football team had played well last season.

2. The man is believed to have been killed by terrorists.

3. The company is thought to be planning a new advertising campaign.

4. The President was reported to have suffered a heart attack.

5. The man is alleged to have been driving at 110 miles an hour.

6. The expedition is known to have reached the South Pole in May.

7. There is said to be a secret tunnel between them.

8. She is considered to have been the best singer that Australia has ever produced.

9. The weather is expected to be good tomorrow.

10. The Prime Minister and his wife are believed to have separated.

Kết luận nào sau đây đúng ?

A. Hai góc kề nhau có tổng số đo bằng 90 độ

B. Hai góc phụ nhau có tổng số đo bằng 180 độ

C. Hai góc bù nhau có tổng số đo bằng 90 độ

D. Hai góc bù nhau có tổng số đo bằng 180 độ

Kết luận nào sau đây đúng ?

A. Hai góc kề nhau có tổng số đo bằng 90 độ

B. Hai góc phụ nhau có tổng số đo bằng 180 độ

C. Hai góc bù nhau có tổng số đo bằng 90 độ

D. Hai góc bù nhau có tổng số đo bằng 180 độ

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mình nghĩ nên đẩy ý b) lên trước vì đã tính AC đâu mà có tỉ số :D

a) Áp dụng định lí Pythagoras cho ΔvuôngABC ta có :

BC² = AB² + AC²

=> \(AC=\sqrt{BC^2-AB^2}=\sqrt{15^2-9^2}=12\left(cm\right)\)

b) Tỉ số hai đoạn thẳng AB và AC : AB/AC = 9/12 = 3/4

c) Vì CD là phân giác của ^C nên theo tính chất đường phân giác trong tam giác ta có : \(\frac{AD}{AC}=\frac{BD}{BC}\)

Áp dụng tính chất dãy tí số bằng nhau ta có : \(\frac{AD}{AC}=\frac{BD}{BC}=\frac{AD+BD}{AC+BC}=\frac{AB}{AC+BC}=\frac{9}{12+15}=\frac{1}{3}\)

=> \(\hept{\begin{cases}\frac{AD}{AC}=\frac{1}{3}\\\frac{BD}{BC}=\frac{1}{3}\end{cases}}\Rightarrow\hept{\begin{cases}AD=\frac{1}{3}AC=4\left(cm\right)\\BC=\frac{1}{3}BC=5\left(cm\right)\end{cases}}\)