下列程序对加号进行了重载，划线部分的语句是______。
#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 tl,Triangle t2)

______

;
void main( )

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