# Arc length formula

Today we are going to find the formula that allows us to calculate the arc length given a function *f*(*x*). This is quite intuitive, so you should have no problem!

This is the formula we are going to get at the end of this post. Although it may look crazy and you may be wondering how the heck can d*y* and d*x* be under the square root, the formula is actually very easy to understand: d*y*/d*x* means “derivative of *y*, i.e. of the function *f*(*x*), therefore you take the derivative and then you square it. Let’s say the function is *f*(*x*)=*x*²: the derivative is 2*x*, and once you square it you get 4*x*²; then you can proceed and solve the integral regurarly.

Now, imagine you approximate the area under a parabola with rectangles, like this:

What’s left is small red regions that look a lot like right triangles. You can see it better here:

The width of each triangle is called d*x*, since it is an infinitely small portion. It can also be called *Δx*, which indicates that it’s a “variation” of *x*. But since we are talking about integrals, that delta *x* is extremely small, so

The same is for d*y*, which is the height:

*ΔL* is the hypotenuse of the right triangle and, again, it is so small that

We know, from the pythagorean theorem, that

In our case *a* is d*x*, *b* is d*y* and *c* is d*L*, therefore

Since we want to find *L*, we can take the integral on both sides (the integral of d*L*, i.e. of 1 with respect to *L*, is *L*):

You can understand this also by thinking that *L* is given by the sum of all d*L*‘s, hence the integral. One *ΔL* doesn’t approximate the curve very well, but many of it do, and to have many of them they need to be extremely small, therefore d*L*.

We know that we can’t integrate with respect to nothing; also, how can we know what d*x* and d*y* are? We can’t! BUT, when you need something, you can simply multiply by it, without forgetting to divide by it as well so that the expression stays the same. We need the d*x*, right? This means we will multiply everything by d*x*/d*x*:

We want just one d*x* in front of the function, therefore we can move the d*x* that is at the bottom inside the square root, but first we have to square it.

Nothing has changed, we are just manipulating things in order to get what we want. At this point we can divide (d*x*)² and (d*y*)² by (d*x*)²:

or

We are almost there!

This gives you the formula to find the arc length, but if you have an interval, this can be written as

or, in a more familiar and compact way, using the definite integral

and this is the formula!

I really hope you liked this post and that everything was clear! If you have any questions, doubts, suggestions, leave a comment and I will be happy to reply!