The opening unit showed us that calculus certainly has its practical aspects. Calculus can help us find the extreme values of a function, compute volumes, probabilities, and a host of other scientific quantities.

It comes as a bit of a pleasant surprise that calculus can also resolve ancient philosophical riddles like **Zeno's paradox of motion**. This unit will guide you to a basic understanding of the paradox and how it relates to **infinite series**. We'll spoil the ending by telling you *how* calculus resolves the paradox, but the details won't be set in place until the penultimate chapter of our course.

Only basic algebra is needed for this unit.

Achilles is the “fleetest foot of all mortals”—meaning he's a pretty fast runner. One day, a snail named Zeno challenges him to a race. “Achilles,” says the snail, “I'll wager the Golden Fleece that I can beat you in a race if you but give me a head start.”

Achilles, knowing he would look pretty good in that Fleece, responds, “You’re on! How can I possibly lose?”

Achilles waits at the starting tree as Zeno makes his way to one half of the total 200-meter distance to the finish line at the grey stone.

Let’s assume Achilles runs at a constant speed of 10 meters per second. We’ll also assume that Zeno runs at a rate of 1 meter per second, which is super fast for a snail!

After 10 seconds, where are Achilles and Zeno?

At this point in the race, Achilles starts to see why Zeno the snail was so certain he could win with a head start. By the time Achilles reaches a mark where Zeno was before, he still has farther to go.

When Achilles reaches the 111 meter mark, where is Zeno?

It looks like the time it takes Achilles to catch up with Zeno is \( 10+ 1 + 0.1 + 0.01 + \cdots \) seconds, where the dots represent an *infinite* number of terms.

This problem comes from the Greek philosopher by the name of Zeno and is meant as a thought experiment. The heart of Zeno's paradox is the impossibility of completing an infinite sum. This seems perfectly reasonable. After all, no matter how fast you are, adding two numbers together takes some amount of time. So it seems like adding an infinite number of numbers should take an infinite amount of time!

It took the invention of calculus, in particular, **infinite series**, to put Zeno firmly to rest. We'll need much more machinery before we can show how calculus does this, but for the time being let's go ahead and figure out exactly how much time it takes Achilles to catch up to the snail.

The resolution of Zeno's paradox comes from showing that \[ 10 + 1 + 0.1 + 0.01 + \dots = 11.\bar{1} = \frac{100}{9}.\]
In particular, we have to make sense of what it even means to sum an infinite number of numbers. This is precisely what the chapter on **infinite series** does and more.

There, we'll not only demonstrate this particular sum, but we'll also connect such infinite sums to topics like quantum theory, differential equations, and one of the most mysterious and useful formulas of all mathematics: Euler's formula.

The key notion underlying the entire chapter on infinite series (indeed, the entire course!) is the **limit**. In short, even though we can't add an infinite number of numbers, we can add a *finite* number of terms and then take the limit as the size of the sum goes to \( \infty.\)

We certainly have a long road ahead of us! But unlike Achilles, we're not in a race, and we should plan on enjoying the journey.

In the next unit, we'll continue our introduction to the basic ideas underlying our course by looking at one of its most important applications: computing the **instantaneous rate of change** of a function.

In trying to resolve Zeno's paradox of motion, we caught a glimpse of the foundational idea of the entire subject, the **limit**. When investigating rates of change, we'll also be unavoidably led to limits once more: limits form the unifying thread of this entire chapter.

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