有以下程序：#include ＜iostream＞#include ＜math＞using namespace stdclass point{private: double x double ypublic: 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. x -b. x)*(a. x-b. x))}int main (){ point p1 (1,2) point p2(5,2) cout＜＜distance (p1, p2)＜＜end1 return 0} 程序运行后的输出结果是