Pick a point in the interior of a Quadrilateral which is not a Parallelogram. Join this point to each of the four Vertices, then the Locus of points for which the sum of opposite Triangle areas is half the Quadrilateral Area is the line joining the Midpoints and of the Diagonals.
See also Diagonal (Polygon), Midpoint, Quadrilateral
References
Honsberger, R. More Mathematical Morsels. Washington, DC: Math. Assoc. Amer., pp. 174-175, 1991.