((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)) < d*d

(Math.Pow(x1-x2,2)+Math.Pow(y1-y2,2)) < (d*d);

// assuming p1 and p2 data types
Point p1, p2;
// distanc can be calculated as follows
double distance = Point.Subtract(p2, p1).Length;

// assuming p1 and p2 data types
Point p1, p2;
// distanc can be calculated as follows
double distanceSquared = (p2 - p1).LengthSquared;

private static double GetDistance(double x1, double y1, double x2, double y2)
{
   return Math.Sqrt(Math.Pow((x2 - x1), 2) + Math.Pow((y2 - y1), 2));
}

double distance = GetDistance(x1, y1, x2, y2)
if(distance <= 5)
{
   //Do stuff
}

double dX = x1 - x2;
double dY = y1 - y2;
double multi = dX * dX + dY * dY;
double rad = Math.Round(Math.Sqrt(multi), 3);

dX = X1 - X2;
dY = Y1 - Y2;

if (dX*dX + dY*dY > (5*5))
{
    //your code
}