Lesson objective: Extend understanding that the scale factor is the same for the perimeters of scale drawings as it is for the dimensions.

Students bring prior knowledge of proportional relationships from Grade 7, Unit 1; 7.G.A.4. This prior knowledge is extended to perimeters of scale drawings as students multiply perimeters of original figures to find perimeters of scale drawings. A conceptual challenge students may encounter is multiplying by a number less than one scales the perimeter down.

The concept is developed through work with a rectangle, which students manipulate to see how the perimeter changes as the dimensions are scaled up and down.

This work helps students deepen their understanding of equivalence because as students study the proportional relationship between original figures and scale drawings, they will extend the idea of equivalent ratios to include figures that are proportionally equivalent.

Students engage in Mathematical Practice 6 (attend to precision) as they ensure that the scale factor includes the proper units, for example if the dimensions of the original figure are in feet and the scale drawing dimensions are in inches, the scale factor must account for that.

**Key vocabulary:**

- original figure
- perimeter
- proportion
- ratio
- scale drawing
- scale factor

**Special materials needed:**

- optional: geo boards and rubber bands