### Parallel Chords: What is this about?

A Mathematical Droodle

What if applet does not run? |

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Copyright © 1996-2018 Alexander Bogomolny

I am grateful to Professor W. McWorter for bringing this problem to my attention. The problem has been posted to the sci.math newsgroup on November 17, 2005. It was found in an old geometry book:

Chords AB and CD are parallel, whereas P and Q are additional points on the same circle. Let X be the intersection of BP and CQ, Y the intersection of AQ and DP. Prove that XY is parallel to AB and CD. |

What if applet does not run? |

An elegant proof was posted the same day:

∠AQC = ∠BPD, as inscribed angles subtending by equal arcs AC and BD. (The latter are enclosed by parallel chords.) This makes the quadrilateral PXYQ cyclic. In its circumcircle, the angles XYP and XQP subtend the same chord XP and are therefore equal. On the other hand, for the same reason but in the given circle angles CQP and CDP are also equal. By transitivity,

I wish to note here that, nice as the problem is, it is only a special case of Pascal's theorem. The points on the given circle form a hexagon, ABPDCQ, whose opposite sides, in general, intersect in points Z, X and Y:

Z = AB∩CD, X = BP∩CQ, Y = AQ∩DP. |

By Pascal's theorem the three points are collinear. This is true even if one of them lies at infinity.

### Pascal and Brianchon Theorems

- Pascal's Theorem
- Pascal in Ellipse
- Pascal's Theorem, Homogeneous Coordinates
- Projective Proof of Pascal's Theorem
- Pascal Lines: Steiner and Kirkman Theorems
- Brianchon's theorem
- Brianchon in Ellipse
- The Mirror Property of Altitudes via Pascal's Hexagram
- Pappus' Theorem
- Pencils of Cubics
- Three Tangents, Three Chords in Ellipse
- MacLaurin's Construction of Conics
- Pascal in a Cyclic Quadrilateral
- Parallel Chords
- Parallel Chords in Ellipse
- Construction of Conics from Pascal's Theorem
- Pascal: Necessary and Sufficient
- Diameters and Chords
- Chasing Angles in Pascal's Hexagon
- Two Triangles Inscribed in a Conic
- Two Triangles Inscribed in a Conic - with Solution
- Two Pascals Merge into One
- Surprise: Right Angle in Circle

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Copyright © 1996-2018 Alexander Bogomolny

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