. Let (A, B, C, D) be a quadrilateral in a Euclidean plane. Assume that the segments [A, C] and [B, D] intersect in their common midpoint X, [A, C]n[B, D] = {x} 3 |A, X|=|C, X| $ |B, X|=|D, X. Show that the lines (AB) and (CD) are parallel.