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2 năm trước
Cho tứ diện ABCD, biết AB vuông góc AC, AB vuông góc BD . Gọi P và Q lần lượt là trung điểm của AB và CD. Chứng minh rằng AB và CD vuông góc với nhau.
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1. Only one person is allowed behind the rider. Two people can sit behind the rider if one of them is sick or under 14 years old. Groups of workers or teenagers on a two-wheel vehicle often break the rule and get into trouble with the police. 2. The speed limit is 40km/h in urban areas and 60km/h on highways and rural roads. The traffic police often have a speed checker. So you will have to pay a fine unless you obey this rule. 3. You have to wear a helmet while riding. It helps protect your head from injuries if accidents happen. However, a lot of people just use helmets because they are scared of the police, not because they care about their safety. 1. Many teens by having more than 2 people behind the rider. 2. The when riding in the city and highways is 40km/h and 60km/h. 3. If you break the rule of speed limit, you will have to 4. Helmets can save you from caused by traffic accidents. 5. Some people wear not because of their safety but to avoid the police.

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