利用多态性编程，创建一个square类，实现求三角形、正方形和圆形面积。方法

//抽象出一个共享的类，定义一个函数求面积的公共界面。再重新定义各面积的求面积

//函数，在主类中创建不同类的对象，并求不同形状的面积 -use polymorphism programming, the creation of a square type, seeking to achieve triangle, square and circular area. Methods / / abstract of a shared type, function for a definition of area public interfaces. Re-definition of the area for the area / / function, in the main category to create different types of objects, and different shapes for the area