Cyclic quadrilateral
In
The word cyclic is from the Ancient Greek κύκλος (kuklos), which means "circle" or "wheel".
All
Special cases
Any
Characterizations
Circumcenter
A convex quadrilateral is cyclic
Supplementary angles
A convex quadrilateral ABCD is cyclic if and only if its opposite angles are
The direct theorem was Proposition 22 in Book 3 of
In 1836 Duncan Gregory generalized this result as follows: Given any convex cyclic 2n-gon, then the two sums of alternate interior angles are each equal to (n-1).[4] This result can be further generalized as follows: lf A1A2...A2n (n > 1) is any cyclic 2n-gon in which vertex Ai->Ai+k (vertex Ai is joined to Ai+k), then the two sums of alternate interior angles are each equal to m (where m = n—k and k = 1, 2, 3, ... is the total turning).[5]
Taking the stereographic projection (half-angle tangent) of each angle, this can be re-expressed,
Which implies that[6]
Angles between sides and diagonals
A convex quadrilateral ABCD is cyclic if and only if an angle between a side and a diagonal is equal to the angle between the opposite side and the other diagonal.[7] That is, for example,
Pascal points
Another necessary and sufficient conditions for a convex quadrilateral ABCD to be cyclic are: let E be the point of intersection of the diagonals, let F be the intersection point of the extensions of the sides AD and BC, let be a circle whose diameter is the segment, EF, and let P and Q be Pascal points on sides AB and CD formed by the circle .
(1) ABCD is a cyclic quadrilateral if and only if points P and Q are collinear with the center O, of circle .
(2) ABCD is a cyclic quadrilateral if and only if points P and Q are the midpoints of sides AB and CD.[2]
Intersection of diagonals
If two lines, one containing segment AC and the other containing segment BD, intersect at E, then the four points A, B, C, D are concyclic if and only if[8]
The intersection E may be internal or external to the circle. In the former case, the cyclic quadrilateral is ABCD, and in the latter case, the cyclic quadrilateral is ABDC. When the intersection is internal, the equality states that the product of the segment lengths into which E divides one diagonal equals that of the other diagonal. This is known as the intersecting chords theorem since the diagonals of the cyclic quadrilateral are chords of the circumcircle.
Ptolemy's theorem
Ptolemy's theorem expresses the product of the lengths of the two diagonals e and f of a cyclic quadrilateral as equal to the sum of the products of opposite sides:[9]: p.25 [2]
where a, b, c, d are the side lengths in order. The converse is also true. That is, if this equation is satisfied in a convex quadrilateral, then a cyclic quadrilateral is formed.
Diagonal triangle
In a convex quadrilateral ABCD, let EFG be the diagonal triangle of ABCD and let be the nine-point circle of EFG. ABCD is cyclic if and only if the point of intersection of the bimedians of ABCD belongs to the nine-point circle .[10][11][2]
Area
The area K of a cyclic quadrilateral with sides a, b, c, d is given by Brahmagupta's formula[9]: p.24
where s, the semiperimeter, is s = 1/2(a + b + c + d). This is a corollary of Bretschneider's formula for the general quadrilateral, since opposite angles are supplementary in the cyclic case. If also d = 0, the cyclic quadrilateral becomes a triangle and the formula is reduced to Heron's formula.
The cyclic quadrilateral has
Four unequal lengths, each less than the sum of the other three, are the sides of each of three non-congruent cyclic quadrilaterals,[13] which by Brahmagupta's formula all have the same area. Specifically, for sides a, b, c, and d, side a could be opposite any of side b, side c, or side d.
The area of a cyclic quadrilateral with successive sides a, b, c, d, angle A between sides a and d, and angle B between sides a and b can be expressed as[9]: p.25
or
or[9]: p.26
where θ is either angle between the diagonals. Provided A is not a right angle, the area can also be expressed as[9]: p.26
Another formula is[14]: p.83
where R is the radius of the circumcircle. As a direct consequence,[15]
where there is equality if and only if the quadrilateral is a square.
Diagonals
In a cyclic quadrilateral with successive vertices A, B, C, D and sides a = AB, b = BC, c = CD, and d = DA, the lengths of the diagonals p = AC and q = BD can be expressed in terms of the sides as[9]: p.25, [16][17]: p. 84
- and
so showing Ptolemy's theorem
According to Ptolemy's second theorem,[9]: p.25, [16]
using the same notations as above.
For the sum of the diagonals we have the inequality[18]: p.123, #2975
Equality holds
Moreover,[18]: p.64, #1639
In any convex quadrilateral, the two diagonals together partition the quadrilateral into four triangles; in a cyclic quadrilateral, opposite pairs of these four triangles are similar to each other.
If ABCD is a cyclic quadrilateral where AC meets BD at E, then[19]
A set of sides that can form a cyclic quadrilateral can be arranged in any of three distinct sequences each of which can form a cyclic quadrilateral of the same area in the same circumcircle (the areas being the same according to Brahmagupta's area formula). Any two of these cyclic quadrilaterals have one diagonal length in common.[17]: p. 84
Angle formulas
For a cyclic quadrilateral with successive sides a, b, c, d, semiperimeter s, and angle A between sides a and d, the trigonometric functions of A are given by[20]
The angle θ between the diagonals that is opposite sides a and c satisfies[9]: p.26
If the extensions of opposite sides a and c intersect at an angle φ, then
where s is the semiperimeter.[9]: p.31
Let denote the angle between sides and , the angle between and , and the angle between and , then:[21]
Parameshvara's circumradius formula
A cyclic quadrilateral with successive sides a, b, c, d and semiperimeter s has the circumradius (the radius of the circumcircle) given by[16][22]
This was derived by the Indian mathematician Vatasseri
Using Brahmagupta's formula, Parameshvara's formula can be restated as
where K is the area of the cyclic quadrilateral.
Anticenter and collinearities
Four line segments, each
If the diagonals of a cyclic quadrilateral intersect at P, and the
The anticenter of a cyclic quadrilateral is the Poncelet point of its vertices.
Other properties
- In a cyclic quadrilateral ABCD, the
- In a cyclic quadrilateral ABCD with circumcenter O, let P be the point where the diagonals AC and BD intersect. Then angle APB is the inscribed angle theorem and the exterior angle theorem.
- There are no cyclic quadrilaterals with rational area and with unequal rational sides in either arithmetic or geometric progression.[26]
- If a cyclic quadrilateral has side lengths that form an arithmetic progression the quadrilateral is also ex-bicentric.
- If the opposite sides of a cyclic quadrilateral are extended to meet at E and F, then the internal angle bisectors of the angles at E and F are perpendicular.[13]
Brahmagupta quadrilaterals
A Brahmagupta quadrilateral[27] is a cyclic quadrilateral with integer sides, integer diagonals, and integer area. All Brahmagupta quadrilaterals with sides a, b, c, d, diagonals e, f, area K, and circumradius R can be obtained by clearing denominators from the following expressions involving rational parameters t, u, and v:
Orthodiagonal case
Circumradius and area
For a cyclic quadrilateral that is also orthodiagonal (has perpendicular diagonals), suppose the intersection of the diagonals divides one diagonal into segments of lengths p1 and p2 and divides the other diagonal into segments of lengths q1 and q2. Then[28] (the first equality is Proposition 11 in Archimedes' Book of Lemmas)
where D is the
or, in terms of the sides of the quadrilateral, as[23]
It also follows that[23]
Thus, according to Euler's quadrilateral theorem, the circumradius can be expressed in terms of the diagonals p and q, and the distance x between the midpoints of the diagonals as
A formula for the area K of a cyclic orthodiagonal quadrilateral in terms of the four sides is obtained directly when combining Ptolemy's theorem and the formula for the area of an orthodiagonal quadrilateral. The result is[29]: p.222
Other properties
- In a cyclic orthodiagonal quadrilateral, the anticenter coincides with the point where the diagonals intersect.[23]
- Brahmagupta's theorem states that for a cyclic quadrilateral that is also orthodiagonal, the perpendicular from any side through the point of intersection of the diagonals bisects the opposite side.[23]
- If a cyclic quadrilateral is also orthodiagonal, the distance from the circumcenter to any side equals half the length of the opposite side.[23]
- In a cyclic orthodiagonal quadrilateral, the distance between the midpoints of the diagonals equals the distance between the circumcenter and the point where the diagonals intersect.[23]
Cyclic spherical quadrilaterals
In spherical geometry, a spherical quadrilateral formed from four intersecting greater circles is cyclic if and only if the summations of the opposite angles are equal, i.e., α + γ = β + δ for consecutive angles α, β, γ, δ of the quadrilateral.[30] One direction of this theorem was proved by Anders Johan Lexell in 1782.[31] Lexell showed that in a spherical quadrilateral inscribed in a small circle of a sphere the sums of opposite angles are equal, and that in the circumscribed quadrilateral the sums of opposite sides are equal. The first of these theorems is the spherical analogue of a plane theorem, and the second theorem is its dual, that is, the result of interchanging great circles and their poles.[32] Kiper et al.[33] proved a converse of the theorem: If the summations of the opposite sides are equal in a spherical quadrilateral, then there exists an inscribing circle for this quadrilateral.
See also
References
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