新建一个基类shape类是一个表示形状的抽象类，area( )为求图形面积的函数。请从shape类派出矩形类(rectangle)、三角形类(triangle)、圆类（circles）、并给出具体的求面积函数。-Create a new class is a base class shape represents the shape of an abstract class, area () function for the sake of the graphic area. Please se