Circles and meet at points and . A circle touches internally at and externally at . A circle touches internally at and externally at .
Prove that the points , , , are either collinear or concyclic.
Circles and meet at points and . A circle touches internally at and externally at . A circle touches internally at and externally at .
Prove that the points , , , are either collinear or concyclic.
Let be the center of and be its radius. Let be the center of and be its radius. Homothety with center at and with a positive coefficient transforms to . Homothety with center at and with a negative coefficient transforms to . Let be the center of the homothety with the negative coefficient which transforms to . By the theorem of three homotheties, lies on the line . Similarly, lies on the line . It is evident that lies on the segment and .


Show that . Let be the intersection point (different from ) of the line and . The power of point with respect to is equal to , on the other hand, it is equal to . From homothety's properties we have . Then , i.e. . Since , we have
Similarly, we obtain that . Therefore, if , , , and do not lie on the same line, then they lie on the same circle.