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Ba người đi xe đẹp đều xuất phát từ a về b.người thứ nhất đt với v1=10km/h.sau 15' người thứ 2 suất phát với v2=13km/h.người 3 đi sau người 2 30'.sau khi gặp người 1,người 3 đi thêm 30'thì cách đều 2 người.tìm vận tốc người 3
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One of the most important discoveries of the nineteenth century was a method of using natural gas for cooking and heating. Large amounts of natural gas are found in the United States, usually several thousand feet below the surface of the earth. Natural gas is most often found in places where petroleum, the oil from which gasoline is made, is found. Often the natural gas must be removed before the oil itself is reached. For many years after natural gas was first discovered, it was thought to have no value. Finally, however, people began to understand its uses and to find ways of storing it and of moving it from place to place. Today natural gas is stored in large tanks and used for lighting, cooking, and heating. In many ways natural gas is one of our finest fuels. It can be used for cooking without making the room hot. It is cheap and can be moved easily from one place to another through long pipelines, some of which are hundreds of miles in length. * Questions : 1. When was the use of gas for cooking and heating first discovered? 2. How is natural gas stored? 3. What is the good thing about natural gas when it is used for cooking? 4. Is natural gas moved from one place to another in trucks? 5. Did people once waste natural gas?

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