填空题 下列程序对加号进行了重载，划线部分的语句是{{U}} 【15】 {{/U}}。 # 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; }