Let A,B,C,D be four points on a circle, and let M=AC∩BD. If the diagonal lines AC and BD are perpendicular, then the perpendicular from M to any side of the quadrilateral passes through the midpoint of the opposite side. This holds whether M lies inside or outside the circumcircle.
Let A,B,C,D be four points on a circle, and let M=AC∩BD. If the diagonal lines AC and BD are perpendicular, then the perpendicular from M to any side of the quadrilateral passes through the midpoint of the opposite side. This holds whether M lies inside or outside the circumcircle.