输出平面上三角形的面积

#include<iostream.h>
#include<math.h>
class tria;
class cpoint
{
	int x1,y1,x2,y2,x3,y3;
public:
	cpoint(int a1,int b1,int a2,int b2,int a3,int b3)                                     	{
		x1=a1;
		y1=b1;
		x2=a2;
		y2=b2;
		x3=a3;
		y3=b3;
	}
	void change(tria &);

};
class tria
{
	double d1,d2,d3,p;
public:
	tria(int a1,int b1,int a2,int b2,int a3,int b3 )
	{
		d1=sqrt((a1-a2)*(a1-a2)+(b1-b2)*(b1-b2));
		d2=sqrt((a1-a3)*(a1-a3)+(b1-b3)*(b1-b3));
		d3=sqrt((a2-a3)*(a2-a3)+(b2-b3)*(b2-b3));
		p=(d1+d2+d3)/2;
	}
	double mianji()
	{
		return sqrt(p*(p-d1)*(p-d2)*(p-d3));
	}
	friend void cpoint::change(tria &);
};
void cpoint::change(tria &c)
{
	
	cout<<"这个三角形面积为"<<c.mianji()<<endl; 



}
void main()
{
	int a1,b1,a2,b2,a3,b3;
	for(int i=0;;i++)
	{
		cout<<"请输入一个点的信息"<<endl;
	    cin>>a1>>b1;
        cout<<"请输入第二个点的信息"<<endl;
	    cin>>a2>>b2;
	    cout<<"请输入第三个点的信息"<<endl;
	    cin>>a3>>b3;
     	cpoint cp(a1,b1,a2,b2,a3,b3);
	    tria c(a1,b1,a2,b2,a3,b3);
     	cp.change(c);
		int j;
		cout<<"如果要继续输入请按1否则按0"<<endl;
		cin>>j;
		if(j)
			continue;
		else 
			break;
	}

}