下面程序的输出结果是 【15】 。 #include ＜iostream.h＞ #include ＜math.h＞ class point double x; double y; public: point(double a, double b) x=a; y=b; friend double distance(point a, point b) ; ; double distance(point a, point b) return sqrt ((a. x-b.x) * (a. x-b.x)+ (a. y-b. y) * (a. y-b. y) );void main() point p1(1,2); point p2(5,2); cout＜＜distance(p1,p2)＜＜end1;