Let OBCD be the quadrilateral P,Q,R, S be the midpoint off OB,CD,OD and BC.
Let the coordinates of O,B,C,D are (0,0), (x,0), (x,y), (0,y)
Coordinates of midpoint of PQ are
Coordinates of midpoint of RS are
Since, the coordinates of the mid-point of PQ = coordinates of mid-point of RS
Therefore, PQ and RS bisect each other