Khối lượng C có trong 1,5 tấn than là: \(m_C=\dfrac{1,5.95}{100}=1,425\) (g)

\(n_C=\dfrac{m}{M}=\dfrac{1,425}{12}=\dfrac{19}{160}\)(mol)

PTHH: C + O₂ → CO₂

Theo PT: \(n_{O_2}=n_C=\dfrac{19}{160}\)

Khối lượng khí oxi cần dùng là: \(m_{O_2}=n.M=\dfrac{19}{160}.32=3,8\)(g) 

a, Do ABCD là hình chữ nhật nên: AB=CD=10cm; AD=BC=5cm; AB//CD ⇒ ∠ABD=∠BDC(so le trong)

Mà ∠AHB=∠BCD=90^o ⇒ ΔAHB ∼ ΔBCD(g.g)

b, Xét ΔADH và ΔBDA, có: ∠AHD=∠BAD=90^o; ∠D chung

⇒ ΔADH ∼ ΔBDA (g.g) ⇒ \(\dfrac{AD}{HD}=\dfrac{BD}{AD} \Rightarrow AD^2=DH.DB\)

c, Xét ΔDHE và ΔDCB, có: ∠DHE=∠DCB=90^o; ∠D chung

⇒ ΔDHE ∼ ΔDCB (g.g) 

⇒ \(\dfrac{DH}{DE}=\dfrac{DC}{DB}\Rightarrow DE=\dfrac{DH.DB}{DC}=\dfrac{{AD} ^2}{DC}=\dfrac{5^2}{10}=\dfrac{5}{2}\) (cm)

\(\Rightarrow S_{\Delta ADE}=\dfrac{1}{2}.AD.DE=\dfrac{1}{2}.5.\dfrac{5}{2}=\dfrac{25}{4}\) (cm²)

A. Rewrite the sentences. Use a compound word. 

1. It is a contest in which participants have to cook rice.

=> It's a rice-cooking contest.

2. Viet Nam is a country which exports a lot of rice.

=> Viet Nam is a rice-exporting country.

3. It is a contest in which participants have to make a fire.

=> It's a fire-making contest.

4. The United States has a big industry that makes cars

=> The United States has a big car-making industry.

 B. Report the questions Nhi asked Nga. 

5. Nam asked Ba "Where do you live?".

-> Nam asked Ba where he lived.

6. Do you know My Son, Nga?

-> Nhi asked Nga if she knew My Son.

7. Is it far from Ha Noi.

-> Nhi asked Nga if it was far from Hanoi.

8. Lan asked me "Do you do your homework?".

-> Lan asked me if I did my homework.

Đổi: 200ml=0,2l

\(n_{Br_2}=C_M.V=0,5.0.2=0,1\) (mol)

PTHH: \(C_2H_4+Br_2\rightarrow C_2H_4Br_2\)

\(\Rightarrow n_{C_2H_4}=n_{Br_2}=0,1\) (mol)

Thể tích C₂H₄ cần dùng là: \(V_{C_2H_4}=n.22,4=0,1.22,4=2,24\) (l)

a, \(2x^2+5x+3=0\Leftrightarrow (2x+3)(x+1)=0 \Leftrightarrow\left[\begin{array}{} x=-\dfrac{3}{2}\\x=-1 \end{array} \right.\)

b, \((x-3)^2=4\Leftrightarrow (x-3)^2-2^2=0\Leftrightarrow (x-5)(x-1)=0 \Leftrightarrow\left[\begin{array}{} x=5\\x=1 \end{array} \right.\)

a, Vì MA, MB là tiếp tuyến của (O) nên \(\angle MAO=\angle MBO=90^o\)

Suy ra: tứ giác OAMB nội tiếp

b, Xét ΔIAC và ΔIBA, có: ∠I chung, \(\angle IAC=\angle IBA\)

\(\Rightarrow \Delta IAC\sim \Delta IBA(g.g) \Rightarrow \dfrac{IA}{IC}=\dfrac{IB}{IA} \Rightarrow IA^2=IB.IC\)

c, Vì I là trung điểm MA nên \(IM^2=IA^2=IB.IC\Rightarrow \dfrac{IC}{IM}=\dfrac{IM}{IB} \)

\(\Rightarrow \Delta ICM \sim \Delta IMB (c.g.c) \Rightarrow \angle IMC=\angle IBM \) hay \(\angle CMA=\angle IBM\)

b, Xét ΔABE và ΔADB, có: \(\angle ABE=\angle ADB,\angle A\) chung 

⇒ ΔABE ∼ ΔADB (g.g) ⇒ \(\dfrac{AB}{AE}=\dfrac{AD}{AB}\Rightarrow AB^2=AD.AE\)

c, Vì BF là đường kính của (O) nên \(\angle FEB=90^o \Rightarrow FE \ \bot \ EB\) tại E.

ΔABO vuông tại H, có BH là đường cao ⇒ \(AB^2=AH.AO\)

\(\Rightarrow AD.AE=AH.AO \Rightarrow \dfrac{AE}{AH}= \dfrac{AO}{AD} \Rightarrow \Delta AHE \sim \Delta ADO\) (c.g.c)

\(\Rightarrow \angle AEH=\angle AOD \Leftrightarrow \angle HED=\angle AOC=\angle HOC\)

Ta có: \(\angle HEB=\angle HED+\angle DEB=\angle HOC+\angle DCB=90^o\)

\(\Rightarrow HE\ \bot \ EB\); mà \(FE\ \bot \ EB\)

\(\Rightarrow \) E, H, F thẳng hàng.

III. WRITING :

A. Complete the following sentences :

1. A new bridge / build / hometown / next year .

→ A new bridge will be built in this hometown next year

2. Before / tornado / start , villagers / move / safe places .

→  Before the tornado started , villagers had moved to the safe places .

B. Rewrite the sentences with the same meaning :

1. Tom said : '' I will be at the seaside tomorrow ."

→Tom said that he would be at the seaside the following day.

2. My brother doesn't do exercise regularly so he gets sick .

→If my brother did exercise regularly, he wouldn't get sick

36. She hasn't surfed the Internet for two days.

→ The last time she surfed the Internet was two days ago.

37. Penicilin can cure many dangerous diseases.

→ Many dangerous diseases can be cured by penicilin.

38. Mount Everest is higher than any other mountains in the world.

→ Mount Everest is the highest mountain in the world.

39. "I'm working in a restaurant." she said.

→ She said she was working in a restaurant.

40. Scientists have tried very hard to find a cure for AIDS.

→ Enormous effort has been made to find a cure for AIDS.