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thuongpro96
2 năm trước
Giúp em câu này với ạ. Em đang cần hơi gấp ạ. Em cảm ơn Một chùm sáng đơn sắc song song chiếu vuông góc với một mặt phẳng của bản mỏng không khí nằm giữa bản thủy tinh đặt tiếp xúc với mặt cong của thấu kính phẳng – lồi. Bán kính của mặt lồi thấu kính là R = 6,4 m. Quan sát hệ vân tròn Newton trong chùm tia sáng phản xạ, người ta đo được bán kính của 2 vân tối kế tiếp lần lượt là 4 mm và 4,38 mm. Hãy xác định: a) bước sóng của chùm sáng chiếu tới; b) số thứ tự của các vân tối; c) bề dày của lớp không khí ở vị trí vân tối thứ 8.
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It is a common saying that we do not fully value a thing until we lose it. We often value the love and worth of a friend when he has been taken from us by death more than when he was with us in the flesh; it is only when we have left school or college that we understand the greatness of our opportunity of education, which has gone forever; and it is the sick and the ailing who realize the value of good health. When we are young and strong, we cannot imagine what it is like to be weak and ill. We are so used to vigorous health that we take it for granted. The organs of our body work so smoothly that we scarcely know we have lungs and liver, heart and stomach. But when any of these gets upset and gives us pain and sickness, we learn by bitter experience what an unspeakable blessing it is to be well. Loss of health makes us miserable, and a burden to ourselves and our friends. It cripples our efforts so that we cannot accomplish many of the good and great things we might have done. It spoils our life. What must we do to keep our health? We must be moderate in eating and drinking and wise in the choice of plain, wholesome simple food. Gluttony has killed thousands, and strong drink tens of thousands. We must, when young, get plenty of sleep, which is “nature’s sweet restorer”, and not try to burn the candle at both ends. We must live as much as possible in the open air and keep our rooms well ventilated. We must get sufficient and regular physical exercise, and keep our bodies clean. And we must avoid bad habits and secret sins as we avoid the devil, and keep our thoughts clean and our bodies pure. Our ideal must be the sound mind in the sound body. 28. According to the passage, we often appreciate our friends . A. only when they are with us B. only when they are good to us. C. only when they passed away D. only when they live a long way from us. 29. We do not take care of our health because . A. we are always in good health B. most of the diseases can be cured nowadays C. we are so accustomed to good health D. we are so busy making a living 30. Poor health makes us . A. useless B. unable to fulfill our expectations C. a shame to our friends D. fail to become famous 31. According to the author, having plenty of sleep when we are young . A. is wasting our time B. can help us regain our strength C. may cause loss of health D. can reduce our vigour 32. The best title for the passage is . A. Health and Human Aspirations B. Health and Our Success C. How to Keep Our Health D. The Value of Health

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