In a previous lesson, students studied a proof of triangle congruence and used it as a model to write their own proof. In this lesson, they are asked to come up with most of the language for a proof of the AngleSideAngle Triangle Congruence Theorem (MP3). They are explicitly encouraged to use previous work and refer to the template as scaffolding.
Students are again given the opportunity to apply the theorem, proving opposite sides of parallelograms are congruent. Students use the definition: a parallelogram is a quadrilateral with two pairs of opposite sides parallel, alternate interior angles, and congruent triangles in their proof. If students struggled with the question on parallel lines and a transversal on the previous end of unit test, there is an optional activity to remind students that alternate interior angles formed by parallel lines and a transversal are congruent.
If some students use a transformation approach rather than applying the AngleSideAngle Triangle Congruence Theorem in the parallelogram proof, there is an opportunity to discuss how students can make use of repeated reasoning by using the AngleSideAngle Triangle Congruence Theorem rather than coming up with a different transformation proof for every case of two triangles that have two pairs of congruent corresponding angles and one pair of congruent corresponding sides between them (MP8).
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.
Lesson overview

7.1 Warmup: Notice and Wonder: Assertion (5 minutes)
 There is a digital applet in this activity.
 7.2 Activity: Proving the AngleSideAngle Triangle Congruence Theorem (15 minutes)
 7.3 Optional Activity: Find the Missing Angle Measures (5 minutes)

7.4 Activity: What Do We Know For Sure About Parallelograms? (15 minutes)
 Includes "Are you Ready for More?" extension problem
 Lesson Synthesis
 7.5 Cooldown: Both (Pairs of) Sides Now (5 minutes)
Learning goals:
 Justify (in writing) the AngleSideAngle Triangle Congruence Theorem.
 Prove (in writing) opposite sides of a parallelogram are congruent.
Learning goals (student facing):
 Let’s see if we can prove other sets of measurements that guarantee triangles are congruent, and apply those theorems.
Learning targets (student facing):
 I can explain why the AngleSideAngle Triangle Congruence Theorem works.
 I can use the AngleSideAngle Triangle Congruence Theorem in a proof.
Required materials:
 Geometry toolkits
 Internetenabled device
Required preparation:
 Acquire internetenabled devices that can run the applet for the activity Notice and Wonder: Assertion, one for every 2–3 students.
 If technology is not available, there is a paper and pencil alternative, but consider displaying the applet for all to see.
 For the lesson synthesis: Create a display of tips for proving lines, angles, or triangles are congruent using triangle congruence theorems.
Glossary:
 parallelogram  A quadrilateral in which pairs of opposite sides are parallel.
 Access the complete Geometry glossary.
Standards:
 This lesson builds on the standards: CCSS.HSGCO.A.5MS.GCO.5CCSS.HSGCO.B.7MS.GCO.7CCSS.HSGCO.B.8MS.GCO.8CCSS.HSGCO.C.9MS.GCO.9MO.G.CO.A.5MO.G.CO.B.6MO.G.CO.B.7MO.G.CO.C.8
 This lesson builds towards the standards: CCSS.HSGCO.B.8MS.GCO.8MO.G.CO.B.7
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