对于各种几何图形，一般都有求图形的面积、周长等方法。现在有圆、矩形和三角形三种图形，要求通过类来实现求三种图形的面积-For a variety of geometric shapes, in general, seek graphic area, perimeter, and other methods. There are circles, rectangles and triangles three types of graphics required by the class to implement the requirements of the three types of graphics area