Area of triangles associated with a curve II
  It is well known that the area $U$ of the triangle formed by three tangents to a parabola $X$ is half of the area $T$ of the triangle formed by joining their points of contact. In this article, we consider whether this property and similar ones characterizes parabolas. As a result, we present three conditions which are necessary and sufficient for a strictly convex curve in the plane to be an open part of a parabola. 
Dong-Soo Kim, Wonyong Kim, Young Ho Kim and Dae Heui Park
math.DG