1. Introduction
The conics passing through the vertices A,B,C of a triangle and its orthocenter
H is the Poncelet pencil; any conic of this pencil is an equilateral hyperbola. The
isogonal transform of a line through the circumcenter O gives a conic of this pencil
and conversely. We described this pencil in [1] and used it to solve some triangle
constructions in [2].
Here is a brief review of some properties of the conics in this pencil. For a
triangle Δ and an equilateral hyperbola K passing through the vertices of Δ, let C
be the circumcircle of Δ and S the fourth point of intersection of these two conics.
Let S
be the antipodal of S on C. Let L be the line through O parallel to the
Wallace-Simson line of S
. Then the isogonal transform of L is K. The center of
K is denoted Z. The nine point circle of any triangle on the equilateral hyperbola
passes through Z since the same equilateral hyperbola serves for any triangle on it.
Garc´ ıa [4] has recently introduced some elementary triangle constructions which
we will use to give some alternate constructions of some of the data of the conics
in the Poncelet pencil. This provides some new insights into the properties of the
conics in Poncelet’s pencil.