Modeling Proportional Relationships

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SWBAT mathematically represent direct and inverse proportional relationships between variables.

Big Idea

A relationship between two quantities can often be described with a mathematical function. Which type of function is most appropriate is dictated by the scenario.

Warm Up

20 minutes

In this warm up, students take 10 minutes to practice making sense of the relationship between two variables.  Warm-up Party Planning is a short activity in which students consider different combinations of ticket prices and number of paying guests that will cover the costs of a party.   Warm-up Party Planning describes the scenario and asks students to create a graph to depict the relationship.  I include this warm up before the relationship card sort in order to remind students that calculating and plotting ordered pairs can be a useful first step in accurately describing a relationship between two variables.    


Paired Brainstorming

20 minutes

Students work in pairs to illustrate their Relationship Cards and begin to discuss how to sort them.  I print the cards on heavy paper and ask students to illustrate them in order to facilitate sorting.  Depending on time constraints, I may ask students to do this for homework the night before.  

The goal of the activity is to help students see that relationships between quantitative variables can often be expressed mathematically and that different types of relationships are represented by different functions.  In the current unit, we are studying rational functions, so many of the cards contain inversely proportional relationships.  I want my students to see that this type of relationship can be modeled with a rational function.

Pairs-to-Groups Poster Session

50 minutes

Pairs of students will join together to form groups of 4 to plan and create a poster explaining their classification system.  Through their brainstorming efforts, students should recognize that 

  • some relationships are "positive" in the sense that as one of the variables goes up, the other one goes up too.
  • some relationships are "negative" in the sense that as one of the variables goes up, the other one goes down.
  • some relationships are linear, in the sense that when one of the variables doubles, the other variables either doubles (positive relationship) or halves (negative relationship)
  • other relationships are non-linear and there are several different models that may apply, including exponential, quadratic, and inversely proportional.

 Students will create the categories into which the cards will be sorted and there could be a wide variety of answers, but the categories above are typical of what students come up with.  

When the categories have been determined, students make a poster that classifies the relationships mathematically.  They will use the Problem Solving Demo Rubric  to assess their progress on the poster.  Over the next few days, we will begin class with students presenting these posters to their peers.