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Forum Geometricorum Volume 6 (2006) 225–227.

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FORUM GEOM ISSN 1534-1178

A Synthetic Proof and Generalization of Bellavitis’ Theorem Nikolaos Dergiades

Abstract. In this note we give a synthetic proof of Bellavitis’ theorem and then generalizing this theorem, for not only convex quadrilaterals, we give a synthetic geometric proof for both theorems direct and converse, as Eisso Atzema proved, by trigonometry, for the convex case [1]. From this approach evolves clearly the connection between hypothesis and conclusion.

1. Bellavitis’ theorem Eisso J. Atzema has recently given a trigonometric proof of Bellavitis’ theorem [1]. We present a synthetic proof here. Inside a convex quadrilateral ABCD, let the diagonal AC form with one pair of opposite sides angles w1 , w3 . Similarly let the angles inside the quadrilateral that the other diagonal BD forms with the remaining pair of opposite sides be w2 , w4 . Theorem 1 (Bellavitis, 1854). If the side lengths of a convex quadrilateral ABCD satisfy AB · CD = BC · DA, then w1 + w2 + w3 + w4 = 180◦ A

w1 C

F

B

w2

F

w4 w3

B

D

C

A

Figure 1

Proof. If AB = AD then BC = CD and AC is the perpendicular bisector of BD. Hence ABCD is a kite, and it is obvious that w1 + w2 + w3 + w4 = 180◦ . AB CB = CD . If ABCD is not a kite, then from AB · CD = BC · DA, we have AD Hence, C lies on the A-Apollonius circle of triangle ABD. See Figure 1. This Publication Date: September 18, 2006. Communicating Editor: Paul Yiu.

226

N. Dergiades

circle has diameter F F  , where AF and AF  are the internal and external bisectors of angle BAD and CF is the bisector of angle BCD. The reflection of AC in AF meets the Apollonius circle at C . Since arc CF = arc F C  , the point C  is the reflection of C in BD. Similarly the reflection of AC in CF meets the Apollonius circle at A that is the reflection of A in BD. Hence the lines AC , CA are reflections of each other in BD and are met at a point B on BD. So we have w2 + w3 =w2 + ∠BCB  = ∠CB D = ∠AB  D 



w1 + w4 =∠B AD + w4 = ∠BB A.

(1) (2)

From (1) and (2) we get w1 + w2 + w3 + w4 = ∠BB  A + ∠AB  D = 180◦ .  2. A generalization There is actually no need for ABCD to be a convex quadrilateral. Since it is clear that w1 + w2 + w3 + w4 = 180◦ for a cyclic quadrilateral, we consider noncyclic quadrilaterals below. We make use of oriented angles and arcs. Denote by θ(XY, XZ) the oriented angle from XY to XZ. We continue to use the notation w1 = θ(AB, AC), w2 = θ(BC, BD),

w3 = θ(CD, CA), w4 = θ(DA, DB).

Theorem 2. In an arbitrary noncyclic quadrilateral ABCD, the side lengths satisfy the equality AB · CD = BC · DA if and only if w1 + w2 + w3 + w4 = ±180◦ . A

w1

A

B

w2

B

w3 C

Figure 2

C

w4

D

A synthetic proof and generalization of Bellavitis’ theorem

227

Proof. Since ABCD is not a cyclic quadrilateral the lines DA, DB, DC meet the circumcircle of triangle ABC at the distinct points A , B  , C  . The triangle A B  C  is the circumcevian triangle of D relative to ABC. Note that 2w1 =arc BC, 2w2 =arc CC  + arc C  B  , 2w3 =arc C  A, 2w4 =arc AB + arcA B  . From these, w1 + w2 + w3 + w4 = ±180◦ if and only if (arc BC + arc CC  + arc C  A + arc AB) + arc C  B  + arc A B  = ±360◦ . Since arc BC +arc CC  +arc C  A+arc AB = ±360◦ , the above condition holds if and only if arc C  B  = arc B  A . This means that the circumcevian triangle of D is isosceles, i.e., (3) B  A = B  C  .       It is well known that A B C is similar to with the pedal triangle A B C of D. See [2, §7.18] The condition (3) is equivalent to B  A = B  C  . This, in turn, is equivalent to the fact that D lies on the B-Apollonius circle of ABC because for a pedal triangle we know that B  A = DC · sin C = B  C  = DA · sin A or

sin A BC DC = = . DA sin C BA From this we have AB · CD = BC · DA.



References [1] E. J. Atzema, A theorem by Giusto Bellavitis on a class of quadrilaterals, Forum Geom., 6 (2006) 181–185. [2] C. Kimberling, Triangle centers and central triangles, Congressus Numerantium, 129 (1998) 1 – 285. Nikolaos Dergiades: I. Zanna 27, Thessaloniki 54643, Greece E-mail address: [email protected]

A Synthetic Proof and Generalization of Bellavitis ...

Sep 18, 2006 - meets the Apollonius circle at C . Since arc CF = arc FC , the point C is the reflection of C in BD. ... E-mail address: [email protected].

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