im' confuse with this function...how do i call one function so it can be used in another function??

```
#include <iostream>
#include <cmath>
using namespace std;
double pi=3.1416;
double getinput();
double distance(double,double,double,double);
double radian(double);
double circumferences(double);
double area(double);
double diameter(double);
void displayresult(double,double,double,double);
int main()
{
double x1,x2,y1,y2;
double rad,perimeter,areas,dia;
cout<<"Please enter center point of a circle :";
cin>>x1>>y1;
cout<<"Please enter a point on a circle :";
cin>>x2>>y2;
rad=radian(rad);
perimeter=circumferences(perimeter);
areas=area(areas);
displayresult(rad,perimeter,areas,dia);
system("pause");
return 0;
}
/*double getinput()
{
double x1,x2,y1,y2;
cout<<"Please enter center point of a circle ";
cin>>x1>>y1;
cout<<"Please enter a point on a circle ";
cin>>x2>>y2;
}*/
double distance(double x1,double x2,double y1, double y2)
{
double dist;
dist = sqrt(pow(x2-x1,2)+pow(y2-y1,2));
return dist;
}
double radian(double rad)
{
rad=distance(x1,x2,y1,y2);
return rad;
}
double circumferences(double perimeter)
{
//double rad;
perimeter=2*pi*rad;
return perimeter;
}
double area (double areas)
{
//double rad;
areas=pi*pow(rad,2);
return areas;
}
double diameter(double dia)
{
//double rad;
dia=2*rad;
return dia;
}
void displayresult(double rad,double perimeter,double areas,double dia)
{
cout<<"The radian of the cirlce is : "<<rad<<endl;
cout<<"The circumferences of the circle is : "<<perimeter<<endl;
cout<<"The area of the circle is : "<<areas<<endl;
cout<<"The diameter of the circle is : "<<dia<<endl;
return;
}
```

help me please....teach me how to call