Arc Length for MATH 141

Understanding Arc Length

So far, we've used integrals to find areas under curves and volumes of solids. But what if you want to find the length of a curve itself? Imagine you have a curvy road on a map and want to know how far you'd actually drive — that's arc length.

The challenge: curves aren't straight, so you can't just use the distance formula. We need calculus to handle the bending!

The Arc Length Formula

For a smooth curve $y = f(x)$ from $x = a$ to $x = b$:

$$L = \int_a^b \sqrt{1 + \left(\frac{dy}{dx}\right)^2} \, dx$$

Or equivalently:

$$L = \int_a^b \sqrt{1 + [f'(x)]^2} \, dx$$

Where Does This Formula Come From?

The intuition: Approximate the curve with tiny straight line segments, then add them up.

Each tiny piece of the curve has:

• Horizontal change: $dx$
• Vertical change: $dy$

By the Pythagorean theorem, the length of each tiny piece is:

$$ds = \sqrt{(dx)^2 + (dy)^2}$$

Factor out $(dx)^2$:

$$ds = \sqrt{1 + \left(\frac{dy}{dx}\right)^2} \, dx$$

Add up all the pieces (integrate):

$$L = \int_a^b \sqrt{1 + \left(\frac{dy}{dx}\right)^2} \, dx$$

Key Steps for Arc Length Problems

1. Find $\frac{dy}{dx}$ (take the derivative)
2. Square it: $\left(\frac{dy}{dx}\right)^2$
3. Add 1: $1 + \left(\frac{dy}{dx}\right)^2$
4. Take the square root: $\sqrt{1 + \left(\frac{dy}{dx}\right)^2}$
5. Integrate from $a$ to $b$

Arc Length with $x = g(y)$

Sometimes it's easier to express the curve as $x$ as a function of $y$. In this case:

$$L = \int_c^d \sqrt{1 + \left(\frac{dx}{dy}\right)^2} \, dy$$

The formula is the same idea — just swap the roles of $x$ and $y$.

Arc Length for Parametric Curves

If the curve is given parametrically as $x = x(t)$, $y = y(t)$ for $t \in [a, b]$:

$$L = \int_a^b \sqrt{\left(\frac{dx}{dt}\right)^2 + \left(\frac{dy}{dt}\right)^2} \, dt$$

Common Mistakes and Misunderstandings

❌ Mistake: Forgetting to square the derivative

Wrong: $L = \int \sqrt{1 + f'(x)} \, dx$

Why it's wrong: The arc length formula requires the derivative to be squared.

Correct: $L = \int \sqrt{1 + [f'(x)]^2} \, dx$

❌ Mistake: Forgetting the "+1" inside the square root

Wrong: $L = \int \sqrt{[f'(x)]^2} \, dx$

Why it's wrong: The formula comes from Pythagorean theorem: $ds = \sqrt{(dx)^2 + (dy)^2}$. The "1" represents the $(dx)^2$ term after factoring.

Correct: $L = \int \sqrt{1 + [f'(x)]^2} \, dx$

❌ Mistake: Using the wrong variable for $x = g(y)$

Wrong: Using $\frac{dy}{dx}$ and integrating with respect to $x$ when the curve is given as $x = g(y)$.

Why it's wrong: When $x$ is a function of $y$, you need $\frac{dx}{dy}$ and integrate with respect to $y$.

Correct: $L = \int_c^d \sqrt{1 + \left(\frac{dx}{dy}\right)^2} \, dy$

❌ Mistake: Not recognizing perfect squares

Wrong: Trying to integrate $\sqrt{\frac{x^2}{4} + \frac{1}{2} + \frac{1}{4x^2}}$ directly.

Why it's wrong: Many textbook problems are designed so that $1 + (f')^2$ simplifies to a perfect square. Always try to factor first!

Correct: Recognize $\frac{x^2}{4} + \frac{1}{2} + \frac{1}{4x^2} = \left(\frac{x}{2} + \frac{1}{2x}\right)^2$, then integrate the simpler expression.

Tips for Arc Length Problems

1. Check if it's a "nice" problem — textbook problems often simplify to perfect squares
2. Consider which form is easier — sometimes $x = g(y)$ is simpler than $y = f(x)$
3. For verification, use the distance formula on straight lines
4. Units matter — arc length has the same units as $x$ and $y$