,
,
and 
in the above figure are in a harmonic range.
Pappus's Harmonic Theorem
See also
Ceva's Theorem, Harmonic Range, Menelaus' Theorem, Pappus's Centroid Theorem, Pappus Chain, Pappus's Hexagon TheoremExplore with Wolfram|Alpha
References
Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 67-68, 1967.Referenced on Wolfram|Alpha
Pappus's Harmonic TheoremCite this as:
Weisstein, Eric W. "Pappus's Harmonic Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PappussHarmonicTheorem.html