Lesson 7

In this lesson, students compare two functions represented in different ways (graph and table, graph and equation, and table and verbal description). In each case, students use the different representations to find outputs for different inputs. Even though they use different representations, students are looking for the same information about the contexts and need to interpret each representation appropriately.

In a graph, students identify the input on the horizontal axis, then find the corresponding coordinate point on the graph, which lets them read the associated output. In a table, they find the input value in the first row (or column) and read the output value in the second. For functions given by equations, students substitute the input value into the expression on the right side of the equation and compute the corresponding output value on the left. Students also look for inputs corresponding to a given output by trying to reverse these procedures. 

Each representation gives us the ability to find input-output pairs. However, each representation has strengths and weaknesses. Graphs require estimation but easily let us identify important features such as highest point or steepest section. Tables immediately let us find output values but only for limited input values. Equations let us precisely compute outputs for all inputs, but only one at a time. Comparing the different strengths of these representations helps students make decisions about how to use these tools strategically in the future.

Note that this lesson specifically avoids comparisons of linear functions to other linear functions, in order to avoid students associating “function” with only linear relationships. In a later lesson, students revisit some of these ideas and compare linear functions.