In this lesson, students examine situations with quantities that decrease exponentially. They work from an equation to a graph and from a graph to an equation. In both cases, they interpret the different parts of their equation in terms of the situation and use the graph to answer questions.

Students are alerted that sometimes people use the terms exponential growth and exponential decay to distinguish between situations in which the growth factor is greater than or less than 1. Additionally, students learn that when the growth factor is less than 1 (but still positive), people sometimes refer to it as the decay factor.

Like many activities in this unit, the equations and graphs represent actual quantities (the area covered by algae and the luminescence of a glow stick) and are to be interpreted in context (MP2).

Technology isn't required for this lesson, but there are opportunities for students to choose to use appropriate technology to solve problems. We recommend making technology available.