1. Let ABCD be a general (skew) quadrilateral and let P, Q, R, S be the mid-points of the sides AB, BC, C D, D A respectively. Show that PQRS is a parallelogram.
2. In a general tetrahedron, lines are drawn connecting the mid-point of each side with the mid-point of the side opposite. Show that these three lines meet in a point that bisects each of them.