题目

已知：x2﹣y2=12，x+y=3，求2x2﹣2xy的值． 答案：2x2﹣2xy=28． 【分析】 先求出x﹣y=4，进而求出2x=7，而2x2﹣2xy=2x（x﹣y），代入即可得出结论． 【详解】 ∵x2﹣y2=12， ∴（x+y）（x﹣y）=12， ∵x+y=3①， ∴x﹣y=4②， ①+②得，2x=7， ∴2x2﹣2xy=2x（x﹣y）=7×4=28． 【点睛】 本题考查了因式分解的应用，代数值求值，二元一次方程组的特殊解法等，求出x-y=4When you watch TV and play video games you make global(全球的) warming worse! It may seem hard to believe, but when electricity is made, so are greenhouse gases(温室气体). This means that every time you use electricity you help make global warming worse!Cars are also making global warming worse. They burn fossil fuels(化石燃料) in their engines, and send lots of greenhouse gases into the air.Global warming may be a big problem, but we can all help stop it. People can try to drive their cars less. Or even get ones that run on sunlight! You can also help. Let’s try one of these top ideas:(1) Try to use less electricity. Turn off lights, your television, and your computer when you’ve stopped using them. To make electricity, fossil fuels are burned in big factories. But burning fossil fuels also makes greenhouse gases. You should also try to watch less TV.(2) Plant trees. Not only is it a fun thing to do, but it is also a great way to lower the number of greenhouse gases in the air. Trees take carbon dioxide out of the air when they grow.(3) Don’t throw away your rubbish, try to recycle it. If rubbish is not recycled, it is put in the ground. There it rots(腐烂) and makes a greenhouse gas called methane(甲烷).So try to recycle cans, bottles, plastic bags and newspapers. It’ll make you feel great! And it’ll help the Earth.小题1:If we _____, we may make global warming worse.A．use less electricityB．drive a car on sunlightC．plant treesD．throw away rubbish小题2:We may use more electricity by______.A．turning off lightsB．shutting down computersC．turning up CD playersD．turning off TV小题3:______ will make global warming worse.A．MethaneB．CansC．NewspapersD．Bottles小题4:Using less electricity does good to global warming because_______.A．the less electricity is used, the more greenhouse gases are producedB．the more electricity is used, the more greenhouse gases are producedC．more electricity is used, less greenhouse gases are producedD．less electricity is used, more greenhouse gases are produced小题5:Which is the following is NOT TRUE according to the passage?A．Driving cars on sunlight helps to stop global warming.B．When cars burn fossil fuels, they produce greenhouse gases.C．When trees grow, they send carbon dioxide into the air.D．By planting more trees, we can lower the number of greenhouse gases in the air.