 This result still holds if the walls are slanted but still parallel and the "heights" "A", "B", and "h" are measured as distances from the floor along lines parallel to the walls.This can be proved easily using the area formula of a trapezoid and area addition formula.In an ellipse, the semi-latus rectum (the distance from a focus to the ellipse along a line parallel to the minor axis) is the harmonic mean of the maximum and minimum distances of the ellipse from a focus.