圆的解决方案区，弧，周长，列和视锥。
Circular solutions for Area, Arc, Circumference, Column and Cones.
要求：分别用成员函数和友元函数来求圆的面积和周长。
Requirements: member function respectively, and a friend function to find area and circumference of a circle.
要求：分别用成员函数和友元函数来求圆的面积和周长。
Requirements: member function respectively, and a friend function to find area and circumference of a circle.
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