下列程序对加号进行了重载，划线部分的语句是 【15】 。
# include＜iostream. h＞
# include＜math. h＞
class Triangle

int x,y,z;
double area;
public:
Triangle(int i,int j, int k)

double s;
x=i;y=j;z=k;
s=(x+y+z)/2.0;
area=sqrt(s*(s-x)*(s-y)*(s-z));

void disparea()

cout＜＜"Area="＜＜area＜＜endl;

friend double operator+(Triangle t1,Triangle t2)

______

;
void main()

Triangle t1(3,4,5),t2(5,12,13);
double s;
cout＜＜"t1:";t1.disparea()
cout＜＜"t2:";t2.disparea()
s=t1+t2;
tout＜＜"总面积:="＜＜s＜＜endl;