Ta có:

\(\dfrac{x}{10}=\dfrac{y}{5}\Rightarrow\dfrac{x}{20}=\dfrac{y}{10}\left(1\right)\)

\(\dfrac{y}{2}=\dfrac{z}{3}\Rightarrow\dfrac{y}{10}=\dfrac{z}{15}\left(2\right)\)

Từ (1) và (2) suy ra: \(\dfrac{x}{20}=\dfrac{y}{10}=\dfrac{z}{15}\Leftrightarrow\dfrac{2x}{40}=\dfrac{y}{10}=\dfrac{4z}{60}=\dfrac{2x-y+4z}{40-10+60}=\dfrac{270}{90}=3\)

\(\Rightarrow\dfrac{x}{20}=3\Rightarrow x=60\)

\(\Rightarrow\dfrac{y}{10}=3\Rightarrow y=30\)

\(\Rightarrow\dfrac{z}{15}=3\Rightarrow z=45\)

sai r a ạ 

\(\dfrac{3^5.3^7}{3^{11}}=\dfrac{3^{12}}{3^{11}}=3\)

a) \(D=4\sqrt{\dfrac{1}{3}}+5\sqrt{12}-6\sqrt{27}\)

\(=\dfrac{4}{9}\sqrt{3}+5.2\sqrt{3}-6.3\sqrt{3}\)

\(=\dfrac{4}{9}\sqrt{3}+10\sqrt{3}-18\sqrt{3}\)

\(=-\dfrac{68}{9}\sqrt{3}\)

b) \(E=\dfrac{2}{\sqrt{3}-1}-\sqrt{4-2\sqrt{3}}\)

\(=\dfrac{2\left(\sqrt{3}+1\right)}{2}-\sqrt{\left(\sqrt{3}\right)^2-2.\sqrt{3}.1+1^2}\)

\(=\sqrt{3}+1-\sqrt{\left(\sqrt{3}-1\right)^2}\)

\(=\sqrt{3}+1-\left(\sqrt{3}-1\right)\)

\(=\sqrt{3}+1-\sqrt{3}+1=2\)

c) \(F=\dfrac{\sqrt{15}-\sqrt{10}}{\sqrt{3}-\sqrt{2}}+\dfrac{3}{2-\sqrt{5}}\)

 \(=\dfrac{\sqrt{5}\left(\sqrt{3}-\sqrt{2}\right)}{\sqrt{3}-\sqrt{2}}+\dfrac{3\left(2+\sqrt{5}\right)}{-1}\)

\(=\sqrt{5}-6-3\sqrt{5}=-2\sqrt{5}-6\)

1. abroad

2. country

3. difficulties

4. fluent

5. achieved

sao câu 9 lại là a v a

1. A

2. D

3. D

4. B

5. A

6. C

7. B

8. C

9. C

10. B

\(\sqrt{y-2}-3\sqrt{64\left(y-2\right)}+4\sqrt{49\left(y-2\right)}\)

\(=\sqrt{y-2}-3\sqrt{8^2\left(y-2\right)}+4\sqrt{7^2\left(y-2\right)}\)

\(=\sqrt{y-2}-3.8\sqrt{y-2}+4.7\sqrt{y-2}\)

\(=\sqrt{y-2}-24\sqrt{y-2}+28\sqrt{y-2}\)

\(=5\sqrt{y-2}\)

1. I wish I could remember the answers to these questions

2. Nam said that he had to go to Ha Noi the next day

3. It is the first time I have been here

4. The planet which is closest to the earth is Venus

5. They told me that there were flights from Nha Trang to Ha Noi twice a week.

6. If the salary weren't high, the job wouldn't be worth doing.

7. I haven't seen Nam for 3 years.

8. Carol told Peter not to make so much noise.

9. The girl who lived next door to us is very friendly

10. If you don't stop talking, you won't understand the lesson.

\(\sqrt{49-5\sqrt{96}}+\sqrt{49+5\sqrt{96}}\)

\(=\sqrt{49-20\sqrt{6}}+\sqrt{49+20\sqrt{6}}\)

\(=\sqrt{5^2-2.5.2\sqrt{6}+\left(2\sqrt{6}\right)^2}+\sqrt{5^2+2.5.2\sqrt{6}+\left(2\sqrt{6}\right)^2}\)

\(=\sqrt{\left(5-2\sqrt{6}\right)^2}+\sqrt{\left(5+2\sqrt{6}\right)^2}\)

\(=5-2\sqrt{6}+5+2\sqrt{6}=10\)