This problem presents an opportunity for students to think about properties of quadrilaterals, and also
to work on confirming observations through geometric reasoning.
If your state has adopted the Common Core State Standards, this alignment might be helpful:
Geometry: Prove Geometric Theorems
G.CO.11. Prove theorems about parallelograms.
3. Construct viable arguments and critique the reasoning of others.
7. Look for and make use of structure.