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[edit]
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 mutually parallel.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Harmonic_quadrilateral.svg/220px-Harmonic_quadrilateral.svg.png)
- Angles ∠BMC and ∠DMC are equal.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Harmonic_quadrilateral2.svg/220px-Harmonic_quadrilateral2.svg.png)
- The bisectors of the angles at B and D intersect on the diagonal AC.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Harmonic_quadrilateral3.svg/220px-Harmonic_quadrilateral3.svg.png)
- 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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Harmonic_quadrilateral5.svg/220px-Harmonic_quadrilateral5.svg.png)
References[edit]
- ^ Johnson, Roger A. (2007) [1929], Advanced Euclidean Geometry, Dover, p. 100, ISBN 978-0-486-46237-0
Further reading[edit]
- Gallatly, W. "The Harmonic Quadrilateral." §124 in The Modern Geometry of the Triangle, 2nd ed. London: Hodgson, pp. 90 and 92, 1913.