The following formula gives the distance between two points, (x1, y1) and (x2, y2) in the Cartesian plane:

#include "stdafx.h"
#include <iostream>
#include<cmath>
const double PI=3.1416;
double sq=2;
using namespace std;
double distance(double x1, double x2, double y1, double y2);
double raduis(double x1, double x2, double y1, double y2);
double circumference(double r);
double area(double r);
int main()
{
double x1;
double x2;
double y1;
double y2;

   
    cout<<"Enter the x and y cordinates of center of the circle: ";
    cin>>x1>>y1;
   
    cout <<endl<<"Enter the x and y coordinates of a point on the circle: ";
    cin>>x2>>y2;
    cout<<endl<<"Raduis = "<<raduis(x1,x2,y1,y2)<<endl;
    cout<<"Diameter = "<<raduis(x1,x2,y1,y2) + raduis(x1,x2,y1,y2)<<endl;
    cout<<"Circumference = "<<circumference(raduis(x1,x2,y1,y2))<<endl;
    cout<<"Area = "<<area(raduis(x1,x2,y1,y2))<<endl;
        system("pause");
return 0;
}

double distance(double x1, double x2, double y1, double y2)
{        
      return sqrt(pow(x2 - x1, sq) + pow(y2 - y1, sq));                  
}

double raduis(double x1, double x2, double y1, double y2)
{
  return distance(x1, x2, y1, y2);                                  
}

double circumference(double r)
{
   return sq * PI * r;
}

double area(double r)
{
   return PI * pow(r, sq);
}