3.2. Proof of Theorem 2. (1) and (2): Consider any chord passing through A between XX and WW. Since the endpoints of this chord lie on parallel lines, the midpoint of the chord must be halfway between these lines, also forming a parallel line with the two original ones. Particularly, the line that the midpoints fall on is y = −x, from the midpoint of XX to the midpoint of WW. By the same reasoning, the locus of midpoints of any chord passing through A between YY and ZZ is on y = x, from the midpoint of YY to the midpoint of ZZ. (3) and (4): We prove (4); then (3) follows by a similar argument. The equations of lines ZW and WXare y = −x−r and y = x−r, respectively. Consider the endpoint C on ZW of a chord passing through A with coordinates (xC,−xC −r). The point D at which CAintersects WXis uniquely determined, so we can calculate D using the slope of CAand hence the equations of lines CA and WX: