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Cho $a;b>0.$ Chứng minh: \[\dfrac 1a+\dfrac 1b\ge \dfrac{4}{a+b}\]
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1. A visit B. travel C. turn D. come 2.A. tell B. say C. report D. explain 3.A. tell B. say C. research D. convince 4.A. take B. make C. get D. go. 5.A. felt B. feel C. feeling D. feels 6.A. try B. begin C. help D. never 7.A. cure B. treat C. research D. fight 8.A. about B. for C. to D. from 9.A. beat B. took C. defeated D. won 10.A. locking B. putting C. endangering D. limiting B. Read the passage then choose the most suitable answer for each question(1 point) The search for alternative sources of energy has led in various directions. Many communities are burning garbage and other biological waste products to produce electricity. Converting waste products to gases or oil is also an effective way to dispose of wastes. Experimental work is being done to derive synthetic fuels from coal, oil shale, and coal tars. But to date, that process has proven expensive. Other experiments are underway to harness power with giant windmills. Geothermal power, heat from the Earth, is also being tested. Some experts expect utility companies to revive hydroelectric power derived from streams and rivers. Fifty years ago hydro-electric power provided one-third of the electricity used in the United States, but today it supplies only 4 percent. The oceans are another potential source of energy. Scientists are studying ways to convert the energy of ocean currents, tides, and waves to electricity. Experiments are also underway to make use of temperature differences in ocean water to produce energy. 1. Which is the best title for the passage? A. The use of Water products for energy B. The search for Alternative sources of Energy. C. Efficient ways of disposing of waste D. New discoveries in Geothermal power. 2. Fifty years ago one-third of the electricity in the United States was provided by______. A. wind B. water products C. water D. oil 3. In the second paragraph, the phrase “Synthetic fuels” could best be replaced by which of the following______. A. biological fuels B. low burning fuels C. fast burning fuels D. artificial made fuels. 4. Which of the following is not mentioned in the passage as an alternative source of energy? A. Burning of garbage B. Geothermal power C. Synthetic fuels D. Electricity. 5. According to the author, the impracticability of using coal, oil shale and coal tars as source of energy is due to______. A. their being time consuming B. their being money consuming C. the scarcity of sources D. the lack of technology

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