The set of points in the plane that, relative to two non-concentric circles
are points of the same power (cf.
Degree of a point).
The equation of the radical axis is:
The radical axis of two disjoint circles lies outside the circles
and is perpendicular to the line through their centres (it is
sometimes assumed that the radical axis of concentric circles is the line
at infinity). The axis of two intersecting circles is the line
passing through the points of intersection, and the radical axis of
two touching circles is their common tangent. For any
three circles with non-collinear centres the radical axes of
each pair of circles pass through one point (the
radical centre).
Figure: r077090a