In this activity, students use a polynomial identity derived in an earlier lesson, , to derive a formula for the sum of the first terms in a geometric sequence. While this is commonly known as the formula for the sum of a finite geometric series, student-facing language was purposefully written to refer only to sequences since series are not a topic of study in this course.

Students begin by returning to the Koch Snowflake, which was first introduced in a previous unit. Thinking of the snowflake as a single triangle with more triangles added at each iteration following a specific pattern, students make sense of this pattern as a series of shapes, as the number of triangles added at each step, and as a general formula for finding the number of triangles added at any given iteration (MP1). Students are then guided to manipulate a general version of the equation for the sum of all the added triangles into a short, rational formula.

In the following activity, students shift to a non-geometric context about the total number of views of an online video, but are still working with a geometric sequence, using the new formula to get a much shorter expression instead of having to add dozens of different terms together. In each context, students make connections between the structures of the long form of the sum, , and the shorter form of the formula, , using the earlier identity (MP7). In the next lesson, students continue to practice applying the formula to different situations.