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Harmonic quadrilateral

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In Euclidean geometry, a harmonic quadrilateral, or harmonic quadrangle,[1] is a quadrilateral that can be inscribed in a circle (cyclic quadrilateral) in which the products of the lengths of opposite sides are equal. It has several important properties.

Properties

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Let ABCD be a harmonic quadrilateral and M the midpoint of diagonal AC. Then:

  • Tangents to the circumscribed circle at points A and C and the straight line BD either intersect at one point or are parallel. Therefore, the pole of each diagonal is contained in the other diagonal respectively.[2][3]
  • Angles ∠BMC and ∠DMC are equal.
  • The bisectors of the angles at B and D intersect on the diagonal AC.
  • A diagonal BD of the quadrilateral is a symmedian of the angles at B and D in the triangles ∆ABC and ∆ADC.
  • The point of intersection of the diagonals is located towards the sides of the quadrilateral to proportional distances to the length of these sides.
  • The point of intersection of the diagonals minimizes the sum of squares of distances from a point inside the quadrilateral to the quadrilateral sides.[4]
  • Considering the points A, B, C, D as complex numbers, the cross-ratio (ABCD) = −1.[3]

References

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  1. ^ Johnson, Roger A. (2007) [1929], Advanced Euclidean Geometry, Dover, p. 100, ISBN 978-0-486-46237-0
  2. ^ "Some Properties of the Harmonic Quadrilateral". Proposition 7
  3. ^ a b "HarmonicQuad".
  4. ^ "Some Properties of the Harmonic Quadrilateral". Proposition 6

Further reading

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  • Gallatly, W. "The Harmonic Quadrilateral." §124 in The Modern Geometry of the Triangle, 2nd ed. London: Hodgson, pp. 90 and 92, 1913.