Lập niên biểu chiến sự tại Đà Nẵng và Gia Định ?

Cho hình chóp S.ABCD đáy là hình vuông, SA vuông góc với (ABCD)

a) CMR : BC vuông góc với (SAB); CD vuông góc với (SAD)

b) CMR : BD vuông góc với (SAC)

c) Kẻ AE vuông góc với SB. CMR : SB vuông góc với (ADE)

Cho hình chóp S.ABCD đáy là hình vuông, SA vuông góc với (ABCD)

a) CMR : BC vuông góc với (SAB), CD vuông góc với (SAD)

b) CMR : BD vuông góc với (SAC)

c) Kẻ AE vuông góc với SB. CMR: SB vuông góc với (ADE)

Cho tứ diện SABC có tam giác ABC vuông tại B; SA ⊥ (ABC)

a) Chứng minh : BC ⊥ (SAB)

b) Gọi AH là đường cao của ΔSAB. Chứng minh : AH⊥SC

Cho hình chóp S.ABCD, có đáy là hình vuông tâm O. SA ⊥ (ABCD). Gọi H, I, K lần lượt là hình chiếu vuông góc của A trên SB, SC, SD.

a) CMR : BC ⊥ (SAB), CD ⊥ (SAD), BD ⊥ (SAC)

b) CMR : AH, AK cùng vuông góc với SC. Từ đó suy ra 3 đường thẳng AH, AI, AK cùng nằm trong một mặt phẳng.

c) CMR : HK ⊥ (SAC). Từ đó suy ra HK ⊥ AI.

Relative clauses replaced by participles and to infinitives

1. The street which leads to the school is very wide ->

2. The system which is used here is very succesful ->

3. John, who teaches my son, is my neighbor ->

4. Trains which leave from this station take an hour to get to London ->

5. The candidates who are sitting for the exam are all from Vietnam ->

6. We are driving on the road which was built in 1980 ->

7. Customers who complain about the service should see the manager ->

8. The city which was destroyed during the war has now been rebuilt ->

9. My brother, who met you yesterday, works for a big firm ->

10. The vegetable which are sold in this shop are grown without chemicals ->

RELATIVE CLAUSES REPLACED BY PARTICIPLES AND TO INFINITIVES

1. The man who is standing there is a clown ->

2. The envelop which lies on the table has no stamp on it ->

3. Benzene, which was discovered by Faraday, became the starting poin in the manufacture of many dyes, perfumes and explosive ->

4. My grandmother, who is old and sick, never goes out of the house ->

5. The student don't know how to do exercise which were given by the teacher yesterday ->

6. The diagrams which were made by young Faraday were sent to Six Humphry Davy at the end of 1812 ->

7. The gentleman who lives next door to me is a well-known orator ->

8. All the astronauts who are orbiting the earth in space capsules are weightless ->

9. All students who don't hand in their papers will fail in the exam ->

10. I saw many houses that were destroyed by the storm ->