Radius is the length of the line segment in between the circumference and the mid point of the circle. So as the definition of the radius points out to calculate the radius of the circle we would need its midpoint and one point on the circumference. Now considering the point in the circumference to give you one coordinate and the midpoint the other, we can use the formula for the distance to find the distance between these two point, in other words the radius. The formula is:
Distance between two points (radius) = √((X2-X1)2+(Y2-Y1)2)
Distance between two points (radius) = √((X2-X1)2+(Y2-Y1)2)
