Let ABC be a triangle. Let DEF be parallel to AC with D on AB and E on BC. Let FGH be parallel to AB with G on BC and H on AC. Let

,

,

, and

be the radii of the circumcircles

,

,

,

of the triangles ABC, DBE, EFG, and HGC, respectively. Then

, with the plus sign when F is outside of ABC and minus when F is inside.