ProblemA:C++习题抽象基类Description编写一个程序，声明抽象基类Shape，由它派生出3个派生类：Circle(圆形)、Rectangle(矩形)、Triangle(三角形)，用一个函数printArea分别输出以上三者的面积（结果保留两位小数），3个图形的数据在定义对象时给定
Input圆的半径矩形的边长三角形的底与高Output圆的面积矩形的面积三角形的面积SampleInput12
4SampleOutputareaofcircle=498
76areaofrectangle=37
80areaoftriangle=18
90#include#includeusingnamespacestd;classShape{public:virtualdoublearea()const=0;};classCircle:publicShape{public:Circle(doubler):radius(r){}virtualdoublearea()const{return3
14159*radius*radius;};protected:doubleradius;};classRectangle:publicShape{public:Rectangle(doublew,doubleh):width(w),height(h){}virtualdoublearea()const{returnwidth*height;}protected:doublewidth,height;};classTriangle:publicShape{public:Triangle(doublew,doubleh):width(w),height(h){}virtualdoublearea()const{return0
5*width*height;}prote
