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; }