编写一个代表三角形的类。其中，三条边为三角形的属性，并封装有求三角形的面积和周长的方法。并设计构造方法和toString（）方法。-Write a representative of the triangle class. Among them, the three sides of the triangle of property, and the package are asking triangle area and perimeter approach. And design and construction method toString () method.