What you appear to be looking for is the furthest neighbour between points. Writing a brute force solution is trivial, just compare distances between every possible pair of points. As per the link, far better solutions exist.
Edit: Distance between two points is
double distance(double x1,double y1,double x2, double y2)
{
return sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1));
}
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solved c programming- calculation of maximum distance between two different points [closed]