The Midpoint Quadrilateral Theorem
Initializing live version

Requires a Wolfram Notebook System
Interact on desktop, mobile and cloud with the free Wolfram Player or other Wolfram Language products.
The midpoints of the sides of any quadrilateral form a parallelogram. Drag any vertex of the magenta quadrilateral ABCD.
Contributed by: Jay Warendorff (March 2011)
Open content licensed under CC BY-NC-SA
Snapshots
Details
This theorem is also known as Varignon's theorem.
Permanent Citation
"The Midpoint Quadrilateral Theorem"
http://demonstrations.wolfram.com/TheMidpointQuadrilateralTheorem/
Wolfram Demonstrations Project
Published: March 7 2011