Lesson 12

Reasoning about Exponential Graphs (Part 1)

Lesson Narrative

In this lesson, students analyze the graph of an exponential function \(f\) given by \(f(x) = a \boldcdot b^x\). In particular, they study the effect of \(b\) on the shape of the graph. To observe how \(b\) impacts the shape of the graph when \(b> 1\) and when \(0<b<1\), they simultaneously examine several functions of this form, all of which have the same value of \(a\).

Narrow contexts are used in these lessons to encourage greater attention to the growth factors of the different functions, rather than on interpreting the quantities in context. Since students are focusing mainly on understanding how varying \(b\) impacts the graph, they are looking at the structure of an exponential function and its graph (MP7).

One of the activities in this lesson works best when each student has access to devices that can run a Desmos applet because students will benefit from seeing the relationship in a dynamic way.

Learning Goals

Teacher Facing

  • Describe (orally and in writing) the effect of changing $a$ and $b$ on a graph that represents $f(x)=a \boldcdot b^x$.
  • Use equations and graphs to compare exponential functions.

Student Facing

Let’s study and compare equations and graphs of exponential functions.

Required Materials

Required Preparation

Acquire devices that can run Desmos (recommended) or other graphing technology. It is ideal if each student has their own device. (Desmos is available under Math Tools.)

Learning Targets

Student Facing

  • I can describe the effect of changing $a$ and $b$ on a graph that represents $f(x)=a \boldcdot b^x$.
  • I can use equations and graphs to compare exponential functions.

CCSS Standards

Addressing

Glossary Entries

  • exponential function

    An exponential function is a function that has a constant growth factor. Another way to say this is that it grows by equal factors over equal intervals. For example, \(f(x)=2 \boldcdot 3^x\) defines an exponential function. Any time \(x\) increases by 1, \(f(x)\) increases by a factor of 3.