Ellipses Inscribed in Parallelograms

Abstract

We prove that there exists a unique ellipse of minimal eccentricity, E_{I}, inscribed in a parallelogram, D. We also prove that the smallest nonnegative angle between equal conjugate diameters of E_{I} equals the smallest nonnegative angle between the diagonals of D. We also prove that if E_{M} is the unique ellipse inscribed in a rectangle, R, which is tangent at the midpoints of the sides of R, then E_{M} is the unique ellipse of minimal eccentricity, maximal area, and maximal arc length inscribed in R. Let D be any convex quadrilateral. In previous papers, the author proved that there is a unique ellipse of minimal eccentricity, E_{I}, inscribed in D, and a unique ellipse, E_{O}, of minimal eccentricity circumscribed about D. We defined D to be bielliptic if E_{I} and E_{O} have the same eccentricity. In this paper we show that a parallelogram, D, is bielliptic if and only if the square of the length of one of the diagonals of D equals twice the square of the length of one of the sides of D.