（2）以Point為基類，派生出一個Circle(圓)類，增加資料成員r(半徑)，以及求面積的成員函式area，實現其他需要的成員函式，設計main函式完成測試；

程式碼

#include<iostream>
#include<Cmath>
using namespace std;
class Point
{
protected:
    double x,y;
public:
    Point(double xx,double yy):x(xx),y(yy){}
    double getx()
    {
        return x;
    }
    double
 
 gety()
    {
        return y;
    }
    void show1();
};
void Point::show1()
{
    cout<<"圓心座標:"<<"("<<x<<","<<y<<")"<<endl;
}
class Circle:public Point
{
protected:
    double r,area;
public:
    Circle(double xx,double yy,double rr):Point(xx,yy),r(rr){}
    double
 
 getarea();
    void show2();
};
double Circle::getarea()
{
    area=r*r*3.14;
    return area;
}
void Circle::show2()
{
    show1();
    cout<<"面積為:"<<getarea()<<endl;
}
int main()
{
    Circle c(2,3,4);
    c.show2();
    return 0;
}