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(a+b+c)² = a² +b² + c² + 2ab + 2ac + 2bc (a-b+c)² = a² +b² + c² – 2ab + 2ac – 2bc Chứng minh đẳng thức: Giải chi tiết hộ
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Scientists are trying to make the deserts into good land again. They want to bring water to the deserts, so people can live and grow foods. They are learning a lot about the deserts. But more and more of the earth is becoming deserts all the time. Scientists may not change the desert in time. Why is more and more land becoming deserts? Scientists think that people make deserts. People are doing bad things to the earth. Some places on the earth don't have very much rain. But they still don't become deserts. This is because some green plants are growing there. Small green plants and grass are very important to dry places. Plants don't let the hot sun make the earth even drier. Plants don't let the wind blow the soil away. When a little rain falls, the plants hold the water. Without plants, the land can become a desert much more easily. 1. According to the passage, deserts …………………. A. can all be turned into good land soon B. never have any plants or animals in them C. get very little rain D. are made by people 2. The underlined word "They" in the passage refers to …………………. A. scientists B. deserts C. people D. foods 3. Small green plants are very important to dry places because ………………… A. they develop together with grass B. they let the sun make the earth even drier C. they don't help to hold the water D. they don't let the wind blow the earth away 4. Land is becoming deserts little by little because …………………. A. there is not enough rain B. people don't do what scientists wish them to do C. plants can't grow there D. scientists know little about the deserts 5. After reading this, we learn that …………………. A. plants can keep dry land from becoming deserts B. it is good to get rid of the grass in the deserts C. all places without much rain will become deserts D. it is better to grow crops on dry land than to grow grass

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