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Forum Geometricorum Volume 7 (2007) 111–113.

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FORUM GEOM ISSN 1534-1178

Some Powerian Pairs in the Arbelos Floor van Lamoen

Abstract. Frank Power has presented two pairs of Archimedean circles in the arbelos. In each case the two Archimedean circles are tangent to each other and tangent to a given circle. We give some more of these Powerian pairs.

1. Introduction We consider an arbelos with greater semicircle (O) of radius r and smaller semicircle (O1 ) and (O2 ) of radii r1 and r2 respectively. The semicircles (O1 ) and (O) meet in A, (O2 ) and (O) in B, (O1 ) and (O2 ) in C and the line through C perpendicular to AB meets (O) in D. Beginning with Leon Bankoff [1], a number of interesting circles congruent to the Archimedean twin circles has been found associated with the arbelos. These have radii r1rr2 . See [2]. Frank Power [5] has presented two pairs of Archimedean circles in the Arbelos with a definition unlike the other known ones given for instance in [2, 3, 4]. 1 D

M1 M2

A

O1

O

C

O2

B

Figure 1

Proposition 1 (Power [5]). Let M1 and M2 be the ’highest’ points of (O1 ) and (O2 ) respectively. Then the pairs of congruent circles tangent to (O) and tangent to each other at M1 and M2 respectively, are pairs of Archimedean circles. To pairs of Archimedean circles tangent to a given circle and to each other at a given point we will give the name Powerian pairs. Publication Date: June 12, 2007. Communicating Editor: Paul Yiu. 1The pair of Archimedean circles (A ) and (A ), with numbering as in [4], qualifies for what 5a 5b we will later in the paper refer to as Powerian pair, as they are tangent to each other at C and to the circular hull of Archimedes’ twin circles. This however is not how they were originally defined.

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F. M. van Lamoen

2. Three double Powerian pairs 2.1. Let M be the midpoint of CD. Consider the endpoints U1 and U2 of the diameter of (CD) perpendicular to OM . D

M1

U1 M

M2 U2

A

O1

O

C

B

O2

Figure 2

√ Note that OC 2 = (r1 − r2 )2 and as CD = 2 r1 r2 that OD2 = r12 − r1 r2 + r22 and OU12 = r12 + r22 . Now consider the pairs of congruent circles tangent to each other at U1 and U2 and tangent to (O). The radii ρ of these circles satisfy (r1 + r2 − ρ)2 = OU12 + ρ2 from which we see that ρ = r1rr2 . This pair is thus Powerian. By symmetry the other pair is Powerian as well. 2.2. Let T1 and T2 be the points of tangency of the common tangent of (O1 ) and (O2 ) not through C. Now consider the midpoint O of O1 O2 , also the center of the semicircle (O1 O2 ), which is tangent to segment T1 T2 at its midpoint. D

T1

M T2

A

O1

O O C

O2

B

Figure 3

  √ 2 2 As T1 T2 = 2 r1 r2 we see that O T12 = r1 +r + r1 r2 . Now consider the 2 pairs of congruent circles tangent to each other at T1 and tangent to (O1 O2 ). The

Some Powerian pairs in the arbelos

113

radii ρ of these circles satisfy   r1 + r2 + ρ)2 − ρ2 = O T12 2 from which we see that ρ = r1rr2 and this pair is Powerian. By symmetry the pair of congruent circles tangent to each other at T2 and to (O1 O2 ) is Powerian. Remark: These pairs are also tangent to the circle with center O through the point where the Schoch line meets (O). √ 2.3. Note that AD = 2 rr1 , hence √ 2r1 r1 r1 AT1 = AD = √ . r r Now consider the pair of congruent circles tangent to each other at T1 and to the circle with center A through C. The radii of these circles satisfy AT12 + ρ2 = (2r1 − ρ)2

from which we see that ρ = r1rr2 and this pair is Powerian. In the same way the pair of congruent circles tangent to each other at T2 and to the circle with center B through C is Powerian. D

T1 M T2

A

O1

O

C

O2

B

Figure 4

References [1] L. Bankoff, Are the Archimedean circles really twin?, Math. Mag., (19) . [2] C. W. Dodge, T. Schoch, P. Y. Woo and P. Yiu, Those ubiquitous Archimedean circles, Math. Mag., 72 (1999) 202–213. [3] F. M. van Lamoen, Archimedean Adventures, Forum Geom., 6 (2006) 79–96. [4] F. M. van Lamoen, Online catalogue of Archimedean Circles, available at http://home.planet.nl/ lamoen/wiskunde/Arbelos/Catalogue.htm [5] F. Power, Some more Archimedean circles in the Arbelos, Forum Geom., 5 (2005) 133–134. Floor van Lamoen: St. Willibrordcollege, Fruitlaan 3, 4462 EP Goes, The Netherlands E-mail address: [email protected]

Some Powerian Pairs in the Arbelos

Jun 12, 2007 - meet in A, (O2) and (O) in B, (O1) and (O2) in C and the line through C ... pairs of congruent circles tangent to each other at T1 and tangent to ...

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