Formula for an infinite geometric series (Part 9)

I continue this series of posts by considering the formula for an infinite geometric series. Somewhat surprisingly (to students), the formula for an infinite geometric series is actually easier to remember than the formula for a finite geometric series.

One way of deriving the formula parallels yesterday’s post. If a_1, a_2, a_3, \dots are the first terms of an infinite geometric sequence, let

S = a_1 + a_2 + a_3 + \dots

Recalling the formula for an geometric sequence, we know that

a_2 = a_1 r

a_3 = a_1 r^2


Substituting, we find

S = a_1 + a_1 r+ a_1 r^2 \dots

For example, if a_1 = \displaystyle \frac{1}{2} and r = \displaystyle \frac{1}{2}, we have

S = \displaystyle \frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \dots

This is perhaps the world’s most famous infinite series, as this is the subject of Zeno’s paradox. When I teach infinite series in class, I often engage the students by reminding students about Zeno’s paradox and then show them this clip from the 1994 movie I.Q.

This clip is almost always a big hit with my students.

Even after showing this clip, some students resist the idea that an infinite series can have a finite answer. For such students, I use a physical demonstration: I walk half-way across the classroom, then a quarter, and so on… until I walk head-first into a walk at full walking speed. The resulting loud thud usually confirms for students that an infinite sum can indeed have a finite answer.

green line

P.S. PhD Comics recently had a cartoon concerning Zeno’s paradox. Source:


Here’s another one. Source:


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