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代码说明：

  有一个圆环，其中小院半径为2.5，大圆半径为7。编程定义一个circle类，含有私有变量半径r，能够初始化r，计算圆面积。主函数中通过定义2个对象（大圆和小圆）来计算出圆环的面积。(There is a circle, of which the radius is 2.5 and the radius is 7. Programming defines a circle class, which contains the private variable radius R. It can initialize R and calculate the circle area. In the main function, the area of the circle is calculated by defining 2 objects (large circles and small circles).)