Lesson 13
Two Graphs for Each Relationship
Lesson Narrative
In this lesson students focus on the relationship between the graph and the equation of a proportional relationship. They start with an activity designed to help them see all the different ways in which the graph and the equation are connected, for example the relation between a point \((a,b)\) on the graph and the constant of proportionality \(k=\frac{b}{a}\) in the equation and the fact that the point \((1,k)\) on the graph tells you the constant of proportionality. This prepares them for the next two activities where they see two ways to graph a proportional relationship, depending on which quantity goes on which axis. This connects with previous work with tables and equations, and gives students an opportunity to remember the fact that the constants of proportionality in the two ways are reciprocals.
As students connect the structure of an equation with features of the graph, they engage in MP7.
Learning Goals
Teacher Facing
 Coordinate (orally and in writing) tables, graphs, and equations that represent the same proportional relationship.
 Interpret two different graphs that represent the same proportional relationship, but have reversed which quantity is represented on each axis.
 Write an equation to represent a proportional relationship given only one pair of values or one point on the graph.
Student Facing
Let’s use tables, equations, and graphs to answer questions about proportional relationships.
Required Materials
Learning Targets
Student Facing
 I can interpret a graph of a proportional relationship using the situation.
 I can write an equation representing a proportional relationship from a graph.
Glossary Entries

coordinate plane
The coordinate plane is a system for telling where points are. For example. point \(R\) is located at \((3, 2)\) on the coordinate plane, because it is three units to the right and two units up.

origin
The origin is the point \((0,0)\) in the coordinate plane. This is where the horizontal axis and the vertical axis cross.