Lesson objective: Understand that perimeter is a linear attribute of plane figures and can be distinguished from area.

Students bring prior knowledge of area from 3.MD.C. This prior knowledge is extended to solving problems as students distinguish between area and perimeter. A conceptual challenge students may encounter is that perimeter is a linear measure of the outside of a plane figure and area is a measure of the space inside a plane figure.

The concept is developed through work with rectangles, which provide a model for the concepts of area and perimeter in a way students can understand and manipulate.

This work helps students deepen their understanding of number because students will use addition and multiplication to calculate area and perimeter.

Students engage in Mathematical Practice 3 (construct viable arguments and critique the reasoning of others) as they constuct a convincing argument for their position on whether the area or perimeter is larger. Students engage in Mathematical Practice 4 (model with mathematics) as they demonstrate the area and perimeter of rectangles.

**Key vocabulary:**

- area
- perimeter
- length
- width