[b]Varignon's theorem[/b][br][br]Take any quadrilateral in the plane. The midpoints of the sides of an arbitrary quadrilateral form a parallelogram. [br][br]If the quadrangle is convex or reentrant, i.e. not a crossing quadrangle, then the area of the parallelogram is half the area of the quadrangle.

[b]Varignon's theorem[/b][br][br]Take any quadrilateral in the plane. The midpoints of the sides of an arbitrary quadrilateral form a parallelogram. [br][br]If the quadrangle is convex or reentrant, i.e. not a crossing quadrangle, then the area of the parallelogram is half the area of the quadrangle.