The line segments joining the midpoints of the opp. sides of a quadrilateral biset each other. 2. Relevant equations 3. The attempt at a solution When I try to prove this for a parallelogram or a rectangle, it seems really obvious. Any number of ways can show this, by using triangles that are similar (using values of x1 and x2.. y1, y2), or simply just for the fact that since the opp are parallel that they have to be equal because if they weren't, the other set of lines could not be parallel. However, I can't find a way to PROVE that it would work for some other quadrilaterals like trapezoids and some of the funky looking ones. I would appreciate a little bump in the right direction, especially since we dont really deal with proofs in my class, its mostly applied math.