Stupel, M., and D. Fraivert. “The Properties of Straight Lines That Issue from the Point of Intersection of the Diagonals of a Trapezoid, and Are Perpendicular to Its Sides”. Journal of Progressive Research in Mathematics, Vol. 5, no. 4, Oct. 2015, pp. 618-23, http://scitecresearch.com/journals/index.php/jprm/article/view/437.