3:

a: CD vuông góc AD

CD vuông góc SA

=>CD vuông góc (SAD)

b: BC vuông góc AB

BC vuông góc SA

=>BC vuông góc (SAB)

=>(SBC) vuông góc (SAB)

Gì dài z má =))

1 She likes reading books

2 What about going to Dam Sen park?

3 If people pollute the water, a lot of fish won't die

4 Peter is taller than Mary

5 We have lived here for 15 years

6 Peter told Anna that he couldn't go swimming the day after

7 His homework about natural disaster has been finished by him

8 He said that they had done an experiment

9 If Huong tidied her house, her Mom wouldn't feel angry

10 A lot of underwater animals die because of the polluted water

1. She likes reading books.

2. What about going to Dam Sen park?

3. If people pollute the water, a lot of fish will die.

4. Peter is taller than Mary.

5. We have lived here for five years.

6. Peter told Anna that he couldn't go fishing the following day.

7. His homework about natural disasters has been finished (by him).

8. They said that they had done an experiment.

9. If Huong doesn't tidy the house, her mom will feel angry.

10. A lot of underwater animals die because of the polluted water.

thôi chịu huhu

a) Ta có :

\(x - y = 5\)

\(\frac{x}{2}=\frac{y}{3}\)

Áp dụng tính chất dãy tỉ số bằng nhau , ta có :

\(\frac{x}{2}=\frac{y}{3}=\frac{x-y}{2-3}=\frac{5}{-1}=-5\)

\(\Rightarrow\hept{\begin{cases}x=-5 . 2 = -10\\y=-5.3=-15\end{cases}}\)

b) Ta có :

\(x - y = 9\)

\(\frac{x}{-2}=\frac{y}{-5}\)

Áp dụng tính chất dãy tỉ số bằng nhau , ta có :

\(\frac{x}{-2}=\frac{y}{-5}=\frac{x-y}{-2-\left(-5\right)}=\frac{9}{3}=3\)

\(\Rightarrow\hept{\begin{cases}x=3. \left(-2 \right)= -6\\y=3 . \left(-5\right) = -15\end{cases}}\)

c) Ta có :

\(x+y+z=6\)

\(\frac{x}{1}=\frac{y}{2}=\frac{z}{3}\)

Áp dụng tính chất dãy tỉ số bằng nhau , ta có :

\(\frac{x}{1}=\frac{y}{2}=\frac{z}{3}=\frac{x+y+z}{1+2+3}=\frac{6}{6}=1\)

\(\Rightarrow\hept{\begin{cases}x = 1.1=1\\y=1.2=2\\z=1.3=3\end{cases}}\)

1C

2A

3C

4B

5C

6D

7B

8A

9D

10C

1 If I were you I would recycle these plastic carrier bags

2 The pilots suddenly struck, so all flights had to be cancelled

3 The polluted environment causes the death of birds and plants

4 The contaminated food leads to people's poor health