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Cyclic Quadrilateral


CyclicQuadrilateral

A cyclic quadrilateral is a quadrilateral for which a circle can be circumscribed so that it touches each polygon vertex. A quadrilateral that can be both inscribed and circumscribed on some pair of circles is known as a bicentric quadrilateral.

The area of a cyclic quadrilateral is the maximum possible for any quadrilateral with the given side lengths. The opposite angles of a cyclic quadrilateral sum to pi radians (Euclid, Book III, Proposition 22; Heath 1956; Dunham 1990, p. 121). There exists a closed billiards path inside a cyclic quadrilateral if its circumcenter lies inside the quadrilateral (Wells 1991, p. 11).

The area is then given by a special case of Bretschneider's formula. Let the sides have lengths a, b, c, and d, let s be the semiperimeter

 s=1/2(a+b+c+d),
(1)

and let R be the circumradius. Then

A=sqrt((s-a)(s-b)(s-c)(s-d))
(2)
=(sqrt((ac+bd)(ad+bc)(ab+cd)))/(4R),
(3)

the first of which is known as Brahmagupta's formula. Solving for the circumradius in (2) and (3) gives

 R=1/4sqrt(((ac+bd)(ad+bc)(ab+cd))/((s-a)(s-b)(s-c)(s-d))).
(4)

The diagonals of a cyclic quadrilateral have lengths

p=sqrt(((ab+cd)(ac+bd))/(ad+bc))
(5)
q=sqrt(((ac+bd)(ad+bc))/(ab+cd)),
(6)

so that pq=ac+bd.

In general, there are three essentially distinct cyclic quadrilaterals (modulo rotation and reflection) whose edges are permutations of the lengths a, b, c, and d. Of the six corresponding polygon diagonals lengths, three are distinct. In addition to p and q, there is therefore a "third" polygon diagonal which can be denoted r. It is given by the equation

 r=sqrt(((ad+bc)(ab+cd))/(ac+bd)).
(7)

This allows the area formula to be written in the particularly beautiful and simple form

 A=(pqr)/(4R).
(8)

The polygon diagonals are sometimes also denoted p, q, and r.

CyclicQuadRectangle
CyclicQuadGrid

The incenters of the four triangles composing the cyclic quadrilateral form a rectangle. Furthermore, the sides of the rectangle are parallel to the lines connecting the mid-arc points between each pair of vertices (left figure above; Fuhrmann 1890, p. 50; Johnson 1929, pp. 254-255; Wells 1991). If the excenters of the triangles constituting the quadrilateral are added to the incenters, a 4×4 rectangular grid is obtained (right figure; Johnson 1929, p. 255; Wells 1991).

CyclicQuadPoints

Consider again the four triangles contained in a cyclic quadrilateral. Amazingly, the triangle centroids M_i, nine-point centers N_i, and orthocenters H_i formed by these triangles are similar to the original quadrilateral. In fact, the triangle formed by the orthocenters is congruent to it (Wells 1991, p. 44).

A cyclic quadrilateral with rational sides a, b, c, and d, polygon diagonals p and q, circumradius r, and area a is given by a=25, b=33, c=39, d=65, p=60, q=52, r=65/2, and a=1344.

Let AHBO be a quadrilateral such that the angles ∠HAB and ∠HOB are right angles, then AHBO is a cyclic quadrilateral (Dunham 1990). This is a corollary of the theorem that, in a right triangle, the midpoint of the hypotenuse is equidistant from the three vertices. Since M is the midpoint of both right triangles DeltaAHB and DeltaBOH, it is equidistant from all four vertices, so a circle centered at M may be drawn through them. This theorem is one of the building blocks of Heron's derivation of Heron's formula.

CyclicQuadCircumcenter

An application of Brahmagupta's theorem gives the pretty result that, for a cyclic quadrilateral with perpendicular diagonals, the distance from the circumcenter O to a side is half the length of the opposite side, so in the above figure,

 OM_(AB)=1/2CD=CM_(CD)=DM_(CD),
(9)

and so on (Honsberger 1995, pp. 37-38).

CyclicQuadOrthocenter

Let M_(AC) and M_(BD) be the midpoints of the diagonals of a cyclic quadrilateral ABCD, and let P be the intersection of the diagonals. Then the orthocenter of triangle DeltaPM_(AC)M_(BD) is the anticenter T of ABCD (Honsberger 1995, p. 39).

CyclicQuadCircles

Place four equal circles so that they intersect in a point. The quadrilateral ABCD is then a cyclic quadrilateral (Honsberger 1991). For a convex cyclic quadrilateral Q, consider the set of convex cyclic quadrilaterals Q_∥ whose sides are parallel to Q. Then the Q_∥ of maximal area is the one whose polygon diagonals are perpendicular (Gürel 1996).


See also

Bicentric Quadrilateral, Brahmagupta's Theorem, Brahmagupta's Trapezium, Bretschneider's Formula, Butterfly Theorem, Concyclic, Cyclic Polygon, Cyclic Quadrangle, Euler Brick, Heron's Formula, Maltitude, Mid-Arc Points, Nine-Point Center, Orthocenter, Poncelet Transverse, Ptolemy's Theorem, Quadrilateral, Tangential Quadrilateral, Triangle Centroid

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References

Andreescu, T. and Gelca, R. "Cyclic Quadrilaterals." §1.2 in Mathematical Olympiad Challenges. Boston, MA: Birkhäuser, pp. 6-9, 2000.Beyer, W. H. CRC Standard Mathematical Tables, 28th ed. Boca Raton, FL: CRC Press, p. 123, 1987.Dunham, W. Journey through Genius: The Great Theorems of Mathematics. New York: Wiley, 1990.Fuhrmann, W. Synthetische Beweise Planimetrischer Sätze. Berlin, 1890.Gürel, E. Solution to Problem 1472. "Maximal Area of Quadrilaterals." Math. Mag. 69, 149, 1996.Harris, J. W. and Stocker, H. "Quadrilateral of Chords." §3.6.7 in Handbook of Mathematics and Computational Science. New York: Springer-Verlag, p. 85, 1998.Heath, T. L. The Thirteen Books of the Elements, 2nd ed., Vol. 3: Books X-XIII. New York: Dover, 1956.Honsberger, R. More Mathematical Morsels. Washington, DC: Math. Assoc. Amer., pp. 36-37, 1991.Honsberger, R. "Cyclic Quadrilaterals." §4.2 in Episodes in Nineteenth and Twentieth Century Euclidean Geometry. Washington, DC: Math. Assoc. Amer., pp. 35-40, 1995.Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, pp. 182-194, 1929.Wells, D. The Penguin Dictionary of Curious and Interesting Geometry. London: Penguin, pp. 11 and 43-44, 1991.

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Cyclic Quadrilateral

Cite this as:

Weisstein, Eric W. "Cyclic Quadrilateral." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CyclicQuadrilateral.html

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