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On Holditch’s theorem. (English) Zbl 1440.53002

Let \(C\) be a convex curve, and a chord \(h\) of length \(a+b\) be divided into parts of lengths \(a\) and \(b\) by a point \(A\). Let \(C_{a,b}\) denote a curve traced out by the point \(A\) when the chord \(h\) slides around with both endpoints on \(C\). H. Holditch [“Geometrical theorem”, Quart. J. Pure Appl. Math. 2, 38 (1858)] proved that the area of a ring domain bounded by \(C\) and \(C_{a,b}\) is equal to \(\pi ab\).
Holditch’s construction, in which one ring domain was considered, is modified by the authors. In this modification, they deal with a family of pairs of ring domains and obtain a natural geometric generalization. They derive some Crofton-type formula for a ring domain as an application.

MSC:

53A04 Curves in Euclidean and related spaces
52A38 Length, area, volume and convex sets (aspects of convex geometry)
53C65 Integral geometry

References:

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