Cho hình chóp S.ABCD có đáy ABCD là hình vuông cạnh a. Gọi K là trung điểm SB, L là điểm thuộc cạnh SD sao cho   Giao điểm I của KL và mặt phẳng (ABCD). Độ dài BI theo a là

Chia sẻ

Một số câu hỏi khác có thể bạn quan tâm.

• Cho hàm số với là tham số thực. Hàm số có bao nhiêu điểm cực tri ?         

• Read the passage carefully and choose the correct answers
  What picture do you have of the future? Will life in the future be better, worse or the same as now? What do you hope about the future?
  Futurologists predict that life will probably be very different in 2050 in all the fields, from entertainment to technology. First of all, it seems that TV channels will have disappeared. Instead, people will choose a program from a 'menu' and a computer will send the program directly to the television.
  Today, we can use the World Wide Web to read newspaper stories and see pictures on a computer thousands of kilometers away. By 2050, music, films, programs, newspapers and books will come to us by computer.
  In what concerns the environment, water will have become one of our most serious problems. In many places, agriculture is changing and farmers are growing fruit and vegetables to export.  This uses a lot of water. Demand for water will increase ten times between now and 2050 and there could be serious shortages. Some futurologists predict that water could be the cause of war if we don't act now.
  In transport, cars running on new, clean fuels will have computers to control the speed and there won't be any accidents. Today, many cars have computers that tell drivers exactly where they are. By 2050, the computer will control the car and drive it to your destination. On the other hand, space planes will take people halfway around the world in two hours. Nowadays, the United States Shuttle can go into space and land on Earth again. By 2050, space planes will fly all over the world and people will fly from Los Angeles to Tokyo in just two hours.
  In the domain of technology, robots will have replaced people in factories. Many factories already use robots. Big companies prefer robots - they do not ask for pay rises or go on strike, and they work 24 hours a day. By 2050, we will see robots everywhere - in factories, schools, offices, hospitals, shops and homes.
  Last but not least, medicine technology will have conquered many diseases. Today, there are electronic devices that connect directly to the brain to help people hear. By 2050, we will be able to help blind and deaf people see and hear again. Scientists have discovered how to control genes. They have already produced clones of animals. By 2050, scientists will be able to produce clones of people and decide how they look, how they behave and how much intelligence they have.
  (Source: Cambridge English for Schools by Andrew Littlejohn, Diana Hicks)
  Question. The passage mainly discusses _________.
  
• Lòng yêu nước là tình yêu quê hương, đất nước và tinh thần sẵn sàng đem hết khả năng của mình
  
• Tình cảm nào dưới đây là biểu hiện rõ của lòng yêu nước?
  
• Ngành công nghiệp mũi nhọn, hàng năm mang lại nguồn tài chính lớn cho LB Nga là
  
• Read the passage carefully and choose the correct answers
  History books record that the first film with sound was The Jazz Singer in 1927. But sound films, or "talkies", did not suddenly appear after years of silent screenings. From the earliest public performances in 1896, films were accompanied by music and sound effects. These were produced by a single pianist, a small band, or a full-scale orchestra; large movie theatres could buy- sound-effects machines. Research into sound that was reproduced at exactly the same time as the pictures – called "synchronized sound" – began soon after the very first films were shown. With synchronized sound, characters on the movie screen could sing and speak. As early as 1896, the newly invented gramophone, which played a large disc carrying music and dialogue, was used as a sound system. The biggest disadvantage was that the sound and pictures could become unsynchronized if, for example, the gramophone needle jumped or if the speed of the projector changed. This system was only effective for a single song or dialogue sequence. In the "sound-on-film" system, sounds were recorded as a series of marks on celluloid which could be read by an optical sensor. These signals would be placed on the film alongside the image, guaranteeing synchronization. Short feature films were produced in this way as early as 1922. This system eventually brought us "talking pictures".
  Question. The passage is mainly about the______.
  
• Có hai lò xo, một lò xo dãn 4 cm khi treo vật khối lượng m₁ = 2 kg, lò xo kia dãn 1 cm khi treo vật có khối lượng m₂ = 1 kg. Tìm tỉ số k₁/k₂.
  

• Lấy ngẫu nhiên một thẻ từ một hộp chứa thẻ được đánh số từ  đến . Xác suất để lấy được thẻ ghi số chia hết cho  là ?  

• Cho 6,58 gam chất X tác dụng mãnh liệt với 100 gam H₂O tạo ra dung dịch Y. Cho Y tác dụng với một lượng BaCl₂ thấy tạo ra 4,66 gam kết tủa trắng và dung dịch Z. Cho Z tác dụng với kim loại Zn dư thu được 1,792 lít H₂ (đktc) và dung dịch E. Nồng độ phần trăm của chất có trong phân tử khối lớn nhất trong dung dịch E là:         

• Nhận xét nào đúng từ bảng số liệu trên ?