Trong không gian \(Oxyz\), lấy điểm \(C\)trên tia \(Oz\) sao cho \(OC = 1\). Trên hai tia \(Ox,Oy\) lần lượt  lấy hai điểm \(A,B\) thay đổi sao cho \(OA + OB = OC\). Tìm giá trị nhỏ nhất của bán kính mặt cầu ngoại tiếp tứ diện \(O.ABC\)? 

Since the dawn of time, people have found ways to communicate with one another. Smoke signals and tribal drums were some of the earliest forms of communication. Letters, carried by birds or by humans on foot or on horseback, made it possible for people to communicate larger amounts of information between two places. The telegram and telephone set the stage for more modern means of communication. With the invention of the cellular phone, communication itself has become mobile. For you, a cell phone is probably just a device that you and your friends use to keep in touch with family and friends, take pictures, play games, or send text message. The definition of a cell phone is more specific: it is a hand-held wireless communication device that sends and receives signals by way of small special areas called cells. Walkie - talkies, telephones and cell phones are duplex communication devices: They make it possible for two people to talk to each other. Cell phones and walkie-talkies are different from regular phones because they can be used in many different locations. A walkie-talkie is sometimes called a half-duplex communication device because only one person can talk at a time. A cell phone is a full-duplex device because it uses both frequencies at the same time. A walkie-talkie has only one channel. A cell phone has more than a thousand channels. A walkie- talkie can transmit and receive signals across a distance of about a mile. A cell phone can transmit and receive signals over hundreds of miles. In 1973, an electronic company called Motorola hired Martin Cooper to work on wireless communication. Motorola and Bell Laboratories (now AT& T) were in a race to invent the first portable communication device. Martin Cooper won the race and became the inventor of the cell phone. On April 3,1973, Cooper made the first cell phone call to his opponent at AT&T while walking down the streets of New York city. People on the sidewalks gazed at Cooper in amazement. Cooper's phone was called A Motorola Dyna-Tac. It weighed a whopping 2.5 pounds (as compared to today's cell phones that weigh as little as 3 or 4 ounces) After the invention of his cell phone, Cooper began thinking of ways to make the cell phone available to the general public. After a decade, Motorola introduced the first cell phone for commercial use. The early cell phone and its service were both expensive. The cell phone itself cost about $3, 500. In 1977, AT&T constructed a cell phone system and tried it out in Chicago with over 2,000 customers. In 1981, a second cellular phone system was started in the Washington, D.C and Baltimore area. It took nearly 37 years for cell phones to become available for general public use. Today, there are more than sixty million cell phone customers with cell phones producing over thirty billion dollars per years. How heavy is the first cell phone compared to today's cell phones?         

Since the dawn of time, people have found ways to communicate with one another. Smoke signals and tribal drums were some of the earliest forms of communication. Letters, carried by birds or by humans on foot or on horseback, made it possible for people to communicate larger amounts of information between two places. The telegram and telephone set the stage for more modern means of communication. With the invention of the cellular phone, communication itself has become mobile. For you, a cell phone is probably just a device that you and your friends use to keep in touch with family and friends, take pictures, play games, or send text message. The definition of a cell phone is more specific: it is a hand-held wireless communication device that sends and receives signals by way of small special areas called cells. Walkie - talkies, telephones and cell phones are duplex communication devices: They make it possible for two people to talk to each other. Cell phones and walkie-talkies are different from regular phones because they can be used in many different locations. A walkie-talkie is sometimes called a half-duplex communication device because only one person can talk at a time. A cell phone is a full-duplex device because it uses both frequencies at the same time. A walkie-talkie has only one channel. A cell phone has more than a thousand channels. A walkie- talkie can transmit and receive signals across a distance of about a mile. A cell phone can transmit and receive signals over hundreds of miles. In 1973, an electronic company called Motorola hired Martin Cooper to work on wireless communication. Motorola and Bell Laboratories (now AT& T) were in a race to invent the first portable communication device. Martin Cooper won the race and became the inventor of the cell phone. On April 3,1973, Cooper made the first cell phone call to his opponent at AT&T while walking down the streets of New York city. People on the sidewalks gazed at Cooper in amazement. Cooper's phone was called A Motorola Dyna-Tac. It weighed a whopping 2.5 pounds (as compared to today's cell phones that weigh as little as 3 or 4 ounces) After the invention of his cell phone, Cooper began thinking of ways to make the cell phone available to the general public. After a decade, Motorola introduced the first cell phone for commercial use. The early cell phone and its service were both expensive. The cell phone itself cost about $3, 500. In 1977, AT&T constructed a cell phone system and tried it out in Chicago with over 2,000 customers. In 1981, a second cellular phone system was started in the Washington, D.C and Baltimore area. It took nearly 37 years for cell phones to become available for general public use. Today, there are more than sixty million cell phone customers with cell phones producing over thirty billion dollars per years. To whom did Cooper make his first cell phone call?         

Since the dawn of time, people have found ways to communicate with one another. Smoke signals and tribal drums were some of the earliest forms of communication. Letters, carried by birds or by humans on foot or on horseback, made it possible for people to communicate larger amounts of information between two places. The telegram and telephone set the stage for more modern means of communication. With the invention of the cellular phone, communication itself has become mobile. For you, a cell phone is probably just a device that you and your friends use to keep in touch with family and friends, take pictures, play games, or send text message. The definition of a cell phone is more specific: it is a hand-held wireless communication device that sends and receives signals by way of small special areas called cells. Walkie - talkies, telephones and cell phones are duplex communication devices: They make it possible for two people to talk to each other. Cell phones and walkie-talkies are different from regular phones because they can be used in many different locations. A walkie-talkie is sometimes called a half-duplex communication device because only one person can talk at a time. A cell phone is a full-duplex device because it uses both frequencies at the same time. A walkie-talkie has only one channel. A cell phone has more than a thousand channels. A walkie- talkie can transmit and receive signals across a distance of about a mile. A cell phone can transmit and receive signals over hundreds of miles. In 1973, an electronic company called Motorola hired Martin Cooper to work on wireless communication. Motorola and Bell Laboratories (now AT& T) were in a race to invent the first portable communication device. Martin Cooper won the race and became the inventor of the cell phone. On April 3,1973, Cooper made the first cell phone call to his opponent at AT&T while walking down the streets of New York city. People on the sidewalks gazed at Cooper in amazement. Cooper's phone was called A Motorola Dyna-Tac. It weighed a whopping 2.5 pounds (as compared to today's cell phones that weigh as little as 3 or 4 ounces) After the invention of his cell phone, Cooper began thinking of ways to make the cell phone available to the general public. After a decade, Motorola introduced the first cell phone for commercial use. The early cell phone and its service were both expensive. The cell phone itself cost about $3, 500. In 1977, AT&T constructed a cell phone system and tried it out in Chicago with over 2,000 customers. In 1981, a second cellular phone system was started in the Washington, D.C and Baltimore area. It took nearly 37 years for cell phones to become available for general public use. Today, there are more than sixty million cell phone customers with cell phones producing over thirty billion dollars per years. The word "duplex" is closest meaning to _______.         

  Since the dawn of time, people have found ways to communicate with one another. Smoke signals and tribal drums were some of the earliest forms of communication. Letters, carried by birds or by humans on foot or on horseback, made it possible for people to communicate larger amounts of information between two places. The telegram and telephone set the stage for more modern means of communication. With the invention of the cellular phone, communication itself has become mobile. For you, a cell phone is probably just a device that you and your friends use to keep in touch with family and friends, take pictures, play games, or send text message. The definition of a cell phone is more specific: it is a hand-held wireless communication device that sends and receives signals by way of small special areas called cells. Walkie - talkies, telephones and cell phones are duplex communication devices: They make it possible for two people to talk to each other. Cell phones and walkie-talkies are different from regular phones because they can be used in many different locations. A walkie-talkie is sometimes called a half-duplex communication device because only one person can talk at a time. A cell phone is a full-duplex device because it uses both frequencies at the same time. A walkie-talkie has only one channel. A cell phone has more than a thousand channels. A walkie- talkie can transmit and receive signals across a distance of about a mile. A cell phone can transmit and receive signals over hundreds of miles. In 1973, an electronic company called Motorola hired Martin Cooper to work on wireless communication. Motorola and Bell Laboratories (now AT& T) were in a race to invent the first portable communication device. Martin Cooper won the race and became the inventor of the cell phone. On April 3,1973, Cooper made the first cell phone call to his opponent at AT&T while walking down the streets of New York city. People on the sidewalks gazed at Cooper in amazement. Cooper's phone was called A Motorola Dyna-Tac. It weighed a whopping 2.5 pounds (as compared to today's cell phones that weigh as little as 3 or 4 ounces) After the invention of his cell phone, Cooper began thinking of ways to make the cell phone available to the general public. After a decade, Motorola introduced the first cell phone for commercial use. The early cell phone and its service were both expensive. The cell phone itself cost about $3, 500. In 1977, AT&T constructed a cell phone system and tried it out in Chicago with over 2,000 customers. In 1981, a second cellular phone system was started in the Washington, D.C and Baltimore area. It took nearly 37 years for cell phones to become available for general public use. Today, there are more than sixty million cell phone customers with cell phones producing over thirty billion dollars per years. What does the word "gazed" mean?         

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions:

The advent of the Internet may be one of the most important technological developments in recent years. Never before have so many people had access to so many different sources of information. For all of the Internet’s advantages, however, people are currently becoming aware of some of its drawbacks and are looking for creative solutions. Among the current problems, which include a general lack of reliability and numerous security concerns, the most crucial is speed. First of all, the Internet has grown very quickly. In 1990, only a few academics had ever heard of the Internet. In 1996, over 50 million people used it. Every year, the number of people with access to the Internet doubles. The rapid growth has been a problem. The computer systems which run the Internet have not been able to keep up with the demand. Also, sometimes, a request for information must pass through many routing computers before the information can be obtained. A request for information made in Paris might have to go through computers in New York, Los Angeles and Tokyo in order to obtain the required information. Consequently, the service is often slow and unpredictable. Service also tends to be worse when the Internet is busiest - during the business day of the Western Hemisphere - which is also when companies need its service the most. Some people are trying to harness the power of networked computers in such a way as to avoid this problem. In 1995, a group of American universities banded together to form what has come to be known as Internet II. Internet II is a smaller, more specialized system intended for academic use. Since it is more specialized, fewer users are allowed access. Consequently, the time required to receive information has decreased. Businesses are beginning to explore a possible analogue to the Internet II. Many businesses are creating their own “Intranets”. These are systems that can only be used by the members of the same company. In theory, fewer users should translate into a faster system. Intranets are very useful for large national and international companies whose branches need to share information. Another benefit of an Intranet is an increased amount of security. Since only company employees have access to the information on the Intranet, their information is protected from competitors. While there is little doubt that the Internet will eventually be a fast and reliable service, industry and the academic community have taken their own steps toward making more practical global networks.

Question: With which of the following conclusions would the author probably agree?             

  Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions:

The advent of the Internet may be one of the most important technological developments in recent years. Never before have so many people had access to so many different sources of information. For all of the Internet’s advantages, however, people are currently becoming aware of some of its drawbacks and are looking for creative solutions. Among the current problems, which include a general lack of reliability and numerous security concerns, the most crucial is speed. First of all, the Internet has grown very quickly. In 1990, only a few academics had ever heard of the Internet. In 1996, over 50 million people used it. Every year, the number of people with access to the Internet doubles. The rapid growth has been a problem. The computer systems which run the Internet have not been able to keep up with the demand. Also, sometimes, a request for information must pass through many routing computers before the information can be obtained. A request for information made in Paris might have to go through computers in New York, Los Angeles and Tokyo in order to obtain the required information. Consequently, the service is often slow and unpredictable. Service also tends to be worse when the Internet is busiest - during the business day of the Western Hemisphere - which is also when companies need its service the most. Some people are trying to harness the power of networked computers in such a way as to avoid this problem. In 1995, a group of American universities banded together to form what has come to be known as Internet II. Internet II is a smaller, more specialized system intended for academic use. Since it is more specialized, fewer users are allowed access. Consequently, the time required to receive information has decreased. Businesses are beginning to explore a possible analogue to the Internet II. Many businesses are creating their own “Intranets”. These are systems that can only be used by the members of the same company. In theory, fewer users should translate into a faster system. Intranets are very useful for large national and international companies whose branches need to share information. Another benefit of an Intranet is an increased amount of security. Since only company employees have access to the information on the Intranet, their information is protected from competitors. While there is little doubt that the Internet will eventually be a fast and reliable service, industry and the academic community have taken their own steps toward making more practical global networks.

Question: All of the following are advantages of business “Intranets” mentioned in the passage EXCEPT__________  

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions:

The advent of the Internet may be one of the most important technological developments in recent years. Never before have so many people had access to so many different sources of information. For all of the Internet’s advantages, however, people are currently becoming aware of some of its drawbacks and are looking for creative solutions. Among the current problems, which include a general lack of reliability and numerous security concerns, the most crucial is speed. First of all, the Internet has grown very quickly. In 1990, only a few academics had ever heard of the Internet. In 1996, over 50 million people used it. Every year, the number of people with access to the Internet doubles. The rapid growth has been a problem. The computer systems which run the Internet have not been able to keep up with the demand. Also, sometimes, a request for information must pass through many routing computers before the information can be obtained. A request for information made in Paris might have to go through computers in New York, Los Angeles and Tokyo in order to obtain the required information. Consequently, the service is often slow and unpredictable. Service also tends to be worse when the Internet is busiest - during the business day of the Western Hemisphere - which is also when companies need its service the most. Some people are trying to harness the power of networked computers in such a way as to avoid this problem. In 1995, a group of American universities banded together to form what has come to be known as Internet II. Internet II is a smaller, more specialized system intended for academic use. Since it is more specialized, fewer users are allowed access. Consequently, the time required to receive information has decreased. Businesses are beginning to explore a possible analogue to the Internet II. Many businesses are creating their own “Intranets”. These are systems that can only be used by the members of the same company. In theory, fewer users should translate into a faster system. Intranets are very useful for large national and international companies whose branches need to share information. Another benefit of an Intranet is an increased amount of security. Since only company employees have access to the information on the Intranet, their information is protected from competitors. While there is little doubt that the Internet will eventually be a fast and reliable service, industry and the academic community have taken their own steps toward making more practical global networks.  

Question: According to the author, what is one reason why the Internet is sometimes slow?         

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions:

The advent of the Internet may be one of the most important technological developments in recent years. Never before have so many people had access to so many different sources of information. For all of the Internet’s advantages, however, people are currently becoming aware of some of its drawbacks and are looking for creative solutions. Among the current problems, which include a general lack of reliability and numerous security concerns, the most crucial is speed. First of all, the Internet has grown very quickly. In 1990, only a few academics had ever heard of the Internet. In 1996, over 50 million people used it. Every year, the number of people with access to the Internet doubles. The rapid growth has been a problem. The computer systems which run the Internet have not been able to keep up with the demand. Also, sometimes, a request for information must pass through many routing computers before the information can be obtained. A request for information made in Paris might have to go through computers in New York, Los Angeles and Tokyo in order to obtain the required information. Consequently, the service is often slow and unpredictable. Service also tends to be worse when the Internet is busiest - during the business day of the Western Hemisphere - which is also when companies need its service the most. Some people are trying to harness the power of networked computers in such a way as to avoid this problem. In 1995, a group of American universities banded together to form what has come to be known as Internet II. Internet II is a smaller, more specialized system intended for academic use. Since it is more specialized, fewer users are allowed access. Consequently, the time required to receive information has decreased. Businesses are beginning to explore a possible analogue to the Internet II. Many businesses are creating their own “Intranets”. These are systems that can only be used by the members of the same company. In theory, fewer users should translate into a faster system. Intranets are very useful for large national and international companies whose branches need to share information. Another benefit of an Intranet is an increased amount of security. Since only company employees have access to the information on the Intranet, their information is protected from competitors. While there is little doubt that the Internet will eventually be a fast and reliable service, industry and the academic community have taken their own steps toward making more practical global networks.  

Question: The word “harness" in paragraph 4 is closest in meaning to         

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions:

The advent of the Internet may be one of the most important technological developments in recent years. Never before have so many people had access to so many different sources of information. For all of the Internet’s advantages, however, people are currently becoming aware of some of its drawbacks and are looking for creative solutions. Among the current problems, which include a general lack of reliability and numerous security concerns, the most crucial is speed. First of all, the Internet has grown very quickly. In 1990, only a few academics had ever heard of the Internet. In 1996, over 50 million people used it. Every year, the number of people with access to the Internet doubles. The rapid growth has been a problem. The computer systems which run the Internet have not been able to keep up with the demand. Also, sometimes, a request for information must pass through many routing computers before the information can be obtained. A request for information made in Paris might have to go through computers in New York, Los Angeles and Tokyo in order to obtain the required information. Consequently, the service is often slow and unpredictable. Service also tends to be worse when the Internet is busiest - during the business day of the Western Hemisphere - which is also when companies need its service the most. Some people are trying to harness the power of networked computers in such a way as to avoid this problem. In 1995, a group of American universities banded together to form what has come to be known as Internet II. Internet II is a smaller, more specialized system intended for academic use. Since it is more specialized, fewer users are allowed access. Consequently, the time required to receive information has decreased. Businesses are beginning to explore a possible analogue to the Internet II. Many businesses are creating their own “Intranets”. These are systems that can only be used by the members of the same company. In theory, fewer users should translate into a faster system. Intranets are very useful for large national and international companies whose branches need to share information. Another benefit of an Intranet is an increased amount of security. Since only company employees have access to the information on the Intranet, their information is protected from competitors. While there is little doubt that the Internet will eventually be a fast and reliable service, industry and the academic community have taken their own steps toward making more practical global networks.  

Question: The word "analogue" in paragraph 5 most nearly means_____.         

Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions:

The advent of the Internet may be one of the most important technological developments in recent years. Never before have so many people had access to so many different sources of information. For all of the Internet’s advantages, however, people are currently becoming aware of some of its drawbacks and are looking for creative solutions. Among the current problems, which include a general lack of reliability and numerous security concerns, the most crucial is speed. First of all, the Internet has grown very quickly. In 1990, only a few academics had ever heard of the Internet. In 1996, over 50 million people used it. Every year, the number of people with access to the Internet doubles. The rapid growth has been a problem. The computer systems which run the Internet have not been able to keep up with the demand. Also, sometimes, a request for information must pass through many routing computers before the information can be obtained. A request for information made in Paris might have to go through computers in New York, Los Angeles and Tokyo in order to obtain the required information. Consequently, the service is often slow and unpredictable. Service also tends to be worse when the Internet is busiest - during the business day of the Western Hemisphere - which is also when companies need its service the most. Some people are trying to harness the power of networked computers in such a way as to avoid this problem. In 1995, a group of American universities banded together to form what has come to be known as Internet II. Internet II is a smaller, more specialized system intended for academic use. Since it is more specialized, fewer users are allowed access. Consequently, the time required to receive information has decreased. Businesses are beginning to explore a possible analogue to the Internet II. Many businesses are creating their own “Intranets”. These are systems that can only be used by the members of the same company. In theory, fewer users should translate into a faster system. Intranets are very useful for large national and international companies whose branches need to share information. Another benefit of an Intranet is an increased amount of security. Since only company employees have access to the information on the Intranet, their information is protected from competitors. While there is little doubt that the Internet will eventually be a fast and reliable service, industry and the academic community have taken their own steps toward making more practical global networks.  

Question: As it can be inferred from the passage, what benefits does Internet II have over the Internet I?