Một thanh nhôm MN, khối lượng  chuyển động trong từ trường đều và luôn tiếp xúc với hai thanh ray đặt song song cách nhau , nằm ngang, nằm trong mặt phẳng hình vẽ. Từ trường ngược hướng với trọng lực, có độ lớn . Hệ số ma sát giữa thanh nhôm MN và hai thanh ray là . Biết thanh nhôm chuyển động đều và điện trở của mạch không đổi.   Lấy . Thanh nhôm chuyển động về phía

Một lớp học có 40 học sinh gồm 25 nam và 15 nữ. Chọn 3 học sinh tham gia vệ sinh công cộng toàn trường, hỏi có bao nhiêu cách chọn 3 học sinh trong lớp?                                                                 

Một nhóm học sinh có 6 bạn nam và 5 bạn nữ có bao nhiêu cách chọn ra 5 bạn trong đó có 3 bạn nam và 2 bạn nữ?                                                         

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

Since water is the basis of life, composing the greater part of the tissues of all living things, the crucial problem of desert animals is to survive in a world where sources of flowing water are rare. And since man’s inexorable necessity is to absorb large quantities of water at frequent intervals, he can scarcely comprehend that many creatures of the desert pass their entire lives without a single drop. Uncompromising as it is, the desert has not eliminated life but only those forms unable to withstand its desiccating effects. No moist-skinned, water-loving animals can exist there. Few large animals are found. The giants of the North American desert are the deer, the coyote, and the bobcat. Since desert country is open, it holds more swift-footed running and leaping creatures than the tangled forest. Its population is largely nocturnal, silent, filled with reticence, and ruled by stealth. Yet they are not emaciated. Having adapted to their austere environment, they are as healthy as animals anywhere else in the word. The secret of their adjustment lies in the combination of behavior and physiology. None could survive if, like mad dogs and Englishmen, they went out in the midday sun; many would die in a matter of minutes. So most of them pass the burning hours asleep in cool, humid burrows underneath the ground, emerging to hunt only by night. The surface of the sun-baked desert averages around 150 degrees, but 18 inches down the temperature is only 60 degrees.  

Question: We can infer from the passage that ______.         

Một nhóm học sinh có 4 nam và 3 nữ. Có bao nhiêu cách chọn 3 bạn trong đó có đúng một bạn là nữ?                                                                

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

Since water is the basis of life, composing the greater part of the tissues of all living things, the crucial problem of desert animals is to survive in a world where sources of flowing water are rare. And since man’s inexorable necessity is to absorb large quantities of water at frequent intervals, he can scarcely comprehend that many creatures of the desert pass their entire lives without a single drop. Uncompromising as it is, the desert has not eliminated life but only those forms unable to withstand its desiccating effects. No moist-skinned, water-loving animals can exist there. Few large animals are found. The giants of the North American desert are the deer, the coyote, and the bobcat. Since desert country is open, it holds more swift-footed running and leaping creatures than the tangled forest. Its population is largely nocturnal, silent, filled with reticence, and ruled by stealth. Yet they are not emaciated. Having adapted to their austere environment, they are as healthy as animals anywhere else in the word. The secret of their adjustment lies in the combination of behavior and physiology. None could survive if, like mad dogs and Englishmen, they went out in the midday sun; many would die in a matter of minutes. So most of them pass the burning hours asleep in cool, humid burrows underneath the ground, emerging to hunt only by night. The surface of the sun-baked desert averages around 150 degrees, but 18 inches down the temperature is only 60 degrees.

Question: The author mentions all the following as examples of the behavior of desert animals EXCEPT_______.         

Ban chấp hành liên chi đoàn khối 11 có 3 nam, 2 nữ. Cần thành lập một ban kiểm tra gồm 3 người trong đó có ít nhất 1 nữ. Số cách thành lập ban kiểm tra là:                                                                        

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

Since water is the basis of life, composing the greater part of the tissues of all living things, the crucial problem of desert animals is to survive in a world where sources of flowing water are rare. And since man’s inexorable necessity is to absorb large quantities of water at frequent intervals, he can scarcely comprehend that many creatures of the desert pass their entire lives without a single drop. Uncompromising as it is, the desert has not eliminated life but only those forms unable to withstand its desiccating effects. No moist-skinned, water-loving animals can exist there. Few large animals are found. The giants of the North American desert are the deer, the coyote, and the bobcat. Since desert country is open, it holds more swift-footed running and leaping creatures than the tangled forest. Its population is largely nocturnal, silent, filled with reticence, and ruled by stealth. Yet they are not emaciated. Having adapted to their austere environment, they are as healthy as animals anywhere else in the word. The secret of their adjustment lies in the combination of behavior and physiology. None could survive if, like mad dogs and Englishmen, they went out in the midday sun; many would die in a matter of minutes. So most of them pass the burning hours asleep in cool, humid burrows underneath the ground, emerging to hunt only by night. The surface of the sun-baked desert averages around 150 degrees, but 18 inches down the temperature is only 60 degrees.  

Question: According to the passage, one characteristic of animals living in the desert is that         

Một lớp học có 40 học sinh gồm 25 nam và 15 nữ. Chọn 3 học sinh tham gia vệ sinh công cộng toàn trường, hỏi có bao nhiêu cách chọn 3 học sinh trog đó có nhiều nhất 1 học sinh nam?                                                 

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

Since water is the basis of life, composing the greater part of the tissues of all living things, the crucial problem of desert animals is to survive in a world where sources of flowing water are rare. And since man’s inexorable necessity is to absorb large quantities of water at frequent intervals, he can scarcely comprehend that many creatures of the desert pass their entire lives without a single drop. Uncompromising as it is, the desert has not eliminated life but only those forms unable to withstand its desiccating effects. No moist-skinned, water-loving animals can exist there. Few large animals are found. The giants of the North American desert are the deer, the coyote, and the bobcat. Since desert country is open, it holds more swift-footed running and leaping creatures than the tangled forest. Its population is largely nocturnal, silent, filled with reticence, and ruled by stealth. Yet they are not emaciated. Having adapted to their austere environment, they are as healthy as animals anywhere else in the word. The secret of their adjustment lies in the combination of behavior and physiology. None could survive if, like mad dogs and Englishmen, they went out in the midday sun; many would die in a matter of minutes. So most of them pass the burning hours asleep in cool, humid burrows underneath the ground, emerging to hunt only by night. The surface of the sun-baked desert averages around 150 degrees, but 18 inches down the temperature is only 60 degrees.  

Question: Man can hardly understand why many animals live their whole life in the desert, as ________.         

Một hộp đựng 5 viên bi màu xanh, 7 viên bi màu vàng. Có bao nhiêu cách lấy ra 6 viên bi bất kỳ?