Lesson objective: Find the perimeter of a polygon with given side lengths.

This lesson helps to build procedural skill with determining perimeter. Polygon models are used here because they provide a visual that supports the skill of finding perimeter. Mathematical expressions are used because they illustrate the use of efficient addition strategies to find perimeter. This work develops students' understanding that perimeter is the sum of the length of the sides of a polygon.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they explore a variety of addition strategies and understand that no matter what the shape of the polygon, perimeter is always calculated by adding the lengths of the sides in any order.

**Key vocabulary:**

- mathematical expression
- perimeter
- polygon
- side lengths
- unit