The purpose of this lesson is for students to see that the quotient of a pair of side lengths in one triangle will always be equal to the quotient of the corresponding side lengths in a similar triangle (MP8). While this fact is not limited to triangles, this lesson focuses on the particular case of triangles so that students are ready to investigate the concept of slope in upcoming lessons.

The lesson begins with students determining if two triangles are similar given only their side lengths and no drawing, prompting them to attempt finding a common scale factor between corresponding sides. Next, students calculate the quotients of pairs of side lengths in similar triangles and find that the quotients for corresponding pairs of sides are the same. Finally, students use this information to calculate missing side lengths in a set of three similar triangles.