运算符重载例题。设计一个三角形类Triangle，包含三角形三条边长的私有数据成员，另有重载运算符“+”与“-”，以实现求两个三角形对象的面积之和与之差。-Operator overloading examples. Design a triangle class Triangle, containing private data members three sides of the triangle long, otherwise overloaded operators " +"　and " -"　to seek to achieve the object of the area of ​ ​ the two triangles and associated poor.