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宇宙中存在一些离其他恒星较远的、由质量相等的三颗星组成的三星系统,通常可忽略其他星体对它们的引力作用.已观测到稳定的三星系统存在两种基本的构成形式:一种形式是三颗星位于同一直线上,两颗星围绕中央星在半径为R的同一圆轨道上运行;另一种形式是三颗星位于等边三角形的三个顶点上,并沿外接于等边三角形的、半径为R的圆形轨道运行.设每颗星体的质量为m.

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    City people usually think they are a lot smarter than country people. They often laugh at simple country ways. But people do not laugh at country music. It is one of the most popular kinds of music in the United States today.

    Perhaps it is so popular because it is about simple but strong human feelings and events—love, sadness, good times and bad times, it tells real life stories and sounds the way people really talk. As life becomes more and more complicated(复杂的), it is good to hear music about ordinary people.

    Country music, sometimes called country western, comes from two kinds of music. One is the traditional music of the people in the Appalachian Mountains in the eastern United States. The other is traditional cowboy music from the West. The singers usually play the guitars, and in the 1920s they started using electric guitars.

    At first city people said country music was low-class. It was popular mostly in the South. But during World War Ⅱ, thousands of Southerners went to the Northeast and Midwest to work in the factories. They took their music with them. Soldiers from the rest of the country went to army camps in the South. They learned to like country music. Slowly it became popular all over the country.

    Today country music is popular everywhere in the United States and Canada, in small towns and in New York City, among black and white, and among educated and uneducated people. About 1,200 radio stations broadcast country music twenty-four hours a day. People sing it in their languages. The music that started with cowboys and poor farmers is now popular all over the world.