Lesson objective: Understand that side lengths and angle measurements remain unchanged when a figure undergoes a rigid transformation.

Students bring prior knowledge of rigid transformations. This prior knowledge is extended to describing congruence as students analyze the side lengths and angle measurements of shapes before and after they are transformed. A conceptual challenge students may encounter is comparing components of shapes that are shown with a different orientation.

The concept is developed through work with rulers and protractors, which are used to confirm that side lengths and angle measurements are the same.

This work helps students deepen their understanding of equivalence because side lengths and angle measurements remain equal when shapes are rotated, reflected, or translated.

Students engage in Mathematical Practice 5 (Use Appropriate Tools Strategically) as they accurately use rulers and protractors to analyze components of shapes.

**Key vocabulary:**

- congruence
- image
- preimage
- reflections
- rotations
- translations

**Special materials needed:**