材料：2017年10月，央视推出新作中国地理大会《绿水青山看中国》，这是聚焦生态文明的创新之作。节目以山、水、林、田、湖、生命为载体，展现人地关系，感念乡土、乡情。

    田地是我们生活的根，第四期节目通过解读主题“田”，传承弘扬农耕文化。著名学者蒙曼和地理学者张捷跨界合作，在精彩评说中延展信息，升华主题，妙语连珠。张教授点评，五谷丰登的基础是耕地，这是我国宝贵的自然资源，也是重要的战略资源，在十三五规划当中，到2020年我国要严控耕地总数，达到18．65亿亩的耕地红线，我们要对耕地红线有一个清晰的认识，这是影响一个国家安全的重大战略。蒙曼教授追加评论，五谷丰登的出处是《六韬》“风雨时节，五谷丰登，社稷安宁”。首先风雨时节指的风调雨顺，风调雨顺后五谷才能丰登，这之后，我们的社稷，我们的粮食才能都安全，国家才能安全。国家安全是建立在粮食安全的基础上，粮食安全的基础是天与地的和谐运转。

设x₁、x₂是一元二次方程x²+x﹣3=0的两根，则x₁³﹣4x₂²+15等于（　　）

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    John was a kind man. When he was young, he hoped ____(become) a singer. But he didn't have enough money ____ (buy) books about music. And his parents didn't expect him ____ (be) a singer. So his dream didn't come tree. At the age of fifteen, he ____ (leave) his village and found a job in a small town. He worked there for five years. Later his mother became very ill (生病的) and he had to ____ (stop) his work to come back and look after (照顾) her. Three years later his mother died and he wanted to leave his village again and  ____ (work) in other places. However, John couldn't. His father was ill. John decided____ (stay) at home and look after him. Every day he ____ (do) many things for his father. Day by day, John became old. But he still didn't forget his dream to be a singer. Now he has lots of time so he plans to learn to sing at a music school.