Lesson 13
Proofs about Parallelograms
Lesson Narrative
In this lesson, students prove two statements about the diagonals of parallelograms.
 The diagonals of a parallelogram bisect each other.
 If the diagonals of a parallelogram are congruent, then the parallelogram is a rectangle.
Students learn a new strategy for looking for structure (MP7) by working backwards from the statement they are trying to prove to the given statements. Students also encounter a situation where they could use overlapping triangles, which can be challenging. Students learn techniques for redrawing or marking diagrams to help them see more subtle triangles which might be used in congruence proofs. As students prove theorems about parallelograms, they are explicitly practicing proof techniques and looking for structure.
One of the activities in this lesson works best when each student has access to internetenabled devices because students will benefit from seeing the relationship in a dynamic way.
Learning Goals
Teacher Facing
 Justify (orally) and prove (in writing) that the diagonals of a parallelogram bisect each other.
 Prove (orally and in writing) that if the diagonals of a parallelogram are congruent, then the parallelogram is a rectangle.
Student Facing
 Let’s prove theorems about parallelograms.
Required Materials
Required Preparation
Acquire internetenabled devices that can run the applet in Notice and Wonder: Diagonals, one for every 23 students. If technology is not available there is a paper and pencil alternative, but consider displaying the applet for all to see.
Learning Targets
Student Facing
 I can prove theorems about the diagonals of a parallelogram.
CCSS Standards
Glossary Entries

rectangle
A quadrilateral with four right angles.

rhombus
A quadrilateral with four congruent sides.